I think that it is interesting that you hit on this... I have been making a recipe from another site called Vanilla Butter Mint. It uses butter and vanilla custard that to me, gives a perfectly balanced bakery custard taste and mouth feel. Its, not too sweet or perfumey as I have found DDL to be, just rich and savory. But I have never seen butter added to anyone's custard recipes. I was just formulating butter into a VCT recipe I am working on when I ran across your post. The aforementioned recipe is a good shake and vape and an awesome mix after about 5 days. I would be interested in how your mixture develops. I personally don't have TFA's Vanilla Custard and up until now, really wasn't interested in acquiring it...Please keep us posted!
It was a great shake and Vape and really developed some good flavor after a few days. That bottle didn't last but the weekend lol. I made another batch of it when I got home but messed up and added WAY too much custard (read the percentage as the ml lol). When I pulled it back out of the bottle, I don't know if I took out enough, or if it took some of the other ingredients with it, but it wasn't the same. That and I put the remainder that I took out back in the custard bottle not thinking so now that flavor can only be used in that recipe.
I think if anything, I would bring the custard down a little as at higher percentages it gets a dry taste to it. I might try again with 8-9% custard and possibly add some cream. Maybe some FA Vienna cream or TFA Bavarian cream. The butter and graham are perfect and really make the flavor come to life though.
Edit: I also was against the idea of the TFA custard as I felt it was too "spicy" for the custard I was aiming for. I had some on hand for my wife's favorite Boston Cream Cheesecake recipe so I thought I'd change it up from CAP v1 custard for a little lol.