Thanks for the added description; because I am working with the handicap of being completely clueless about Scentsy products, every little bit of additional info helps!
Here is what I am now imagining based on your additional description:1.Defined noticeable "fresh" marshmallow (vs. a "campfire" toasted/caramelized marshmallow)(though
@ceeceeisme recalls a distinct "toasted" marshmallow). "Fresh" does
seem to match most images associated with the Scentsy product.
2. Butter Mint is only a light, subtle, secondary "background note.
Additional thoughts, and considerations, from
@ceeceeisme's previous image, and description (quoted above for easy reference):
The image kind of makes me wonder because I see three things represented: the marshmallow (fresh not toasted, Heath bar/toffee with chocolate, and a caramel/cream or caramel/vanilla swirled candy of some sort. I recall ceecee used a caramel in her first effort; but that was abandoned without comment. And, of course, her vivid recollection of toasted vs. fresh marshmallow. Further, the Scentsy description: "Buttermint and marshmallow make merry beneath an
amber veil," strengthens my belief that there is a caramel, and/or, toffee element in this.
So, taking these two expanded descriptions, I am now thinking:
Primary Flavor:
- Marshmallow. Regular ("fresh"), or Toasted, or some combination of each.
- Vanilla. A vanilla, or vanillin, might be useful here (in small amounts) to bring out more actual flavor to the marshmallow.
Secondary Flavors:
- Peppermint. This appears to be a no-brainer other than "how much."
- Butter/buttercream? This may/may not be necessary; as the caramel, or toffee might contribute the needed "butter" note.
- Caramel. Caramel can add to the warm burnt (caramelized) sugar note which might be part of the "toasted marshmallow" that ceecee is remembering. Caramel quickly can dominate lighter flavors so this will need to be added at extremely low percentages (0.25% or even less?)
- Toffee. Toffee is similar to the caramel in that is can contribute the caramelized sugar and butter notes. Toffee and caramel might be an "either/or," or both. TFA's English toffee intrigues me here; in that it is a full "Heath Bar" concentrate that includes hints of chocolate and nuts. Again; use only in extremely low percentages.
- Vanilla. Vanilla is as much a contributor to smell as it is to taste. I think a vanilla "accentuator" like TFA Vanillin would be beneficial to bring out the preexisting vanilla in the marshmallow and creams. Other vanillas may be considered if vanillin is inadequate (perhaps something light like CAP Vanilla Whipped Cream?).
- Additional Dairy (creams and custards). I would only consider these if the "body" of the e-liquid is still too thin. To me, creams don't have much aroma in and of themselves.
- Ethyl Maltol (EM). This is an ingredient is often a double-edged sword. It can lend to the "sugariness" (not as much "sweet" as "sugar") in a recipe; but it can also mute other flavors (especially subtle ones). I would work without this, and only try if I needed that extra sugar bolus.
- Sweetener (sucralose, stevia, et al). Like the EM; I would only use this as the "final piece" to the puzzle. I think the concentrates will have sufficient sweetness but might be the thing that take the recipe from "good" to "great." But again, only consider adding if the rest of the recipe is as good as it is going to get, and still needs a bit more sweet.
Marshmallows are kind of a weird duck. They are basically made from sugar, corn syrup, gelatin, vanilla, and a touch of salt; yet they have a unique, and very definable, "marshmallow" smell. I really can't explain why, or where it comes from, other than it has to be from the interaction of those ingredients plus heat.
So; how would I go about developing this from scratch? First, since I have identified the marshmallow as being what I want front-and-center, I would focus solely on that and forget about the whole butter mint side of things.
Part of the problem is that Marshmallow is a fairly light/delicate/subtle(?) smell, and can easily be overwhelmed by other, more dominating flavors (kind of like what
@ceeceeisme is concerned about with the mint). This is even more so with marshmallow flavor concentrates. Look at concentrate reviews, and you will see the majority of them seem to use Marshmallow for mouthfeel (as a "thickener"), instead of an actual flavor.
I am
guessing that this is going to develop best by using multiple marshmallow concentrates. Perhaps, something like TFA Marshmallow+FA Marshmallow+FW Toasted Marshmallow (I prefer FW to TFA as it has more of a "caramelization" vs. "burnt char" aspect to it when compared to TFA), to get all the nuances of the Scentsy marshmallow aroma.
To do this, it will be helpful to know how each marshmallow tastes on its own, then I can know what each is contributing to the mix of two, or more, as well as give me a better feel for what percentages I may want to use. Once I understand the individual marshmallow candidates, I can
then start experimenting with the combining of them.
Once I have the marshmallow "base" optimized I can then look at adding additional supporting/accent flavors. I would probably start with just the caramel, or toffee, because this may actually fill out the marshmallow further (or even be a complete flop and tossed from potential players). Again, I am
guessing, but this might meet the "butter" requirements of the overall recipe profile, and eliminate (or at least reduce) the need for a separate butter ingredient.
When I am thinking of caramel, or toffee here, I am thinking of it contributing more of that warm, buttery, smell/taste of cooking caramel/toffee; verses the stronger, more dominating flavor, of the finished candy. Less is definitely going to be better than more here. As little a 0.1% may be the ticket. The possibility of just a hint of chocolate and nut (more "aroma" than "taste." Just the suggestion is what I am after) from TFA English Toffee is stuck in my mind; so that is probably where
I would start.
Then, and only then, would I consider the addition of a further butter, or cream note (and those in minimum quantities too), remember that each addition is also diluting the marshmallow base as well.
Once I get the full, decadent, marshmallow figured out, then it should be a simple matter of adding the mint, and figuring out how much of a presence I want it to have.
Note: It would not surprise me at all, if the total of
all secondary flavors is only 25% (or even less) of the marshmallow base total percentage, in order to let the marshmallow keep center stage. In other words, if I have concentrate(s) equaling 8% to make up my "perfect" marshmallow, all butters, vanillas, mints, and creams will equal 2% or less (2% being 25 percent of 8%). Otherwise I
think you are likely to lose the marshmallow.
That would be my game plan. I all but wiped myself out of TFA Marshmallow on my last mix so it will be some time before I able to play with this further.

DISCLAIMER: This is
ONLY how
I would go about developing it. It is
NOT "
the right way," or "
the best way," it is only what works for
me. Others may share different methods and deserve every bit the same consideration. Find what works best
for you.
Nothing wrong with that at all.

To me, you are simply being smart, to seek input from those with more experience. That is one of the great benefits of a community like ECF; it provides a wonderful network of a wide range of experience. You only deprive yourself by not asking.
Do keep in mind that
other's experience will only go so far though because personal taste, and preference, are so individual. NOTHING can replace your own participation and observation. What may be divine to me could very well be vile to you.
Okay; 48 hour update for the mix I made in post #49:
It has already settled down significantly. I am now getting a noticeable "cool" from the mint on inhale though I am not tasting actual mint (it is very similar to the cool one gets from koolada). On exhale; the mint begins to come out along with the cool, as well as much more tempered butter and custard notes. I am no longer getting nearly as strong a "burning plastic/rubber" though there is still just a hint (from the Toasted Marshmallow perhaps? I expect this to be gone completely before long). The heavy custard and butter are still completely overwhelming any suggestion that there is marshmallow in this; let alone is supposed to be a marshmallow-dominant vape. I
am getting a bit of the caramelization (much better balance compared to original tasting) from the Toasted Marshmallow; but no marshmallow its self. There is something in this that sits unpleasantly "heavy" in what I expected would be a fairly light and "fluffy" profile (of course that very well could go away as it continues to age). Sweetness is good and I see no further need/desire for additional sweeteners at this time.
As this settles down, my
early thoughts are; that the Butter needs to be ramped way back (or even eliminated?) because
I am getting more of a "melted" movie popcorn butter taste than the smell of butter browning in a pan with sugar. I also suspect it of being the chief contributor of the unwanted heaviness. Likewise, I think this might do without the CAP Vanilla Custard; I think this is too dominant of a flavor (heavy, thick, righ, with a touch of "egginess") for what it is supposed to be contributing. CAP Butter Cream might be a better option to replace both. Or, reduce the Butter (down to 1% or even less?), and then add CAP Vanilla Whipped Cream to introduce a little extra vanilla while keeping it light.
I will still give this the full three weeks before passing final judgment. Please remember these are only my
current thoughts as this mix continues to age and develop.