Like 'butter face', but instead of a hot body with an ugly face, the person possesses exceptionally good looks but a small/not so good looking crotch. For men, common terms would be 'pencil dick' or 'baby penis'. Women, terms like 'blue waffle' or the favorite 'stink tuna'.
Jake: Dude, I swear Amanda is so freakin' hot! I'm really considering getting in bed with her tonight.
Dave: Ehh, yeah she's good looking, but I wouldn't bang. Word is she's a butter crotch.
Dave: Yep. Her clitoris looks like chewed up bubble gum. Pretty sick man.