Shake Shack - Marshmallow Peanut Butter Swirl Custard

I never grew up eating Fluffernutter sandwiches, but I feel like this Shake Shack custard definitely took all of the best parts of that sandwich into an insanely delicious custard flavor.  This marshmallow peanut butter swirl custard was awesome, and really managed to highlight both the marshmallow and peanut butter, instead of it tasting too much like one and not the other.  I wonder how this custard would be in an actual sandwich, haha!