Personally, I have decided not to inhale the "custard" flavors from any flavor company. This is what Perfumers Apprentice says about their vanilla custard flavoring.
Vanilla Custard
water soluble
a rich, buttery custard flavor. stronger than our bavarian cream (which has no "Custard" Notes).
*** contains both "Custard" ingredients (acetoin and acetyl propionyl) -
Please see our information section on these ingredients in the Flavorist Corner (lower left of the screen)
****** This flavor should only be used in Food and Cosmetic applications.
****** This flavor (no matter what company makes it) has inhalation risks if it contains either Acetoin or Acetyl Propionyl!
Ingredients: Natural & Artificial Flavors, Propylene Glycol, a small amount of Ethyl Alcohol
You can read additional info they post about custard flavors here:
The Flavorist Workshop