I'm A Banana
Potassium-Rich
If at any time he needs to cook something, it's automatically good. I mean, why bother rolling?
It's pretty fun.
I mean, I'd go with the whole "Taking 10" rules for this. You can automatically, given enough time and no stressful situations, do pretty decent at it (take 10). If you come to a stressful do-or-die situation, though, you've gotta roll.
Because it's really nifty seeing the expert cook roll a 1 and then thinking of an explanation for why it's not up to par ("In your rush to prepare the cake, you forgot to steady it properly, and it fell on the floor. Not having time to bake another one, you added another layer of frosting, and sent it out anyway. The King doesn't really buy your story about 'Hair Cake.'"), or, alternately, seeing them roll a 20 and thinking of the superlative awesomeness that can result ("The dinner is interrupted as an avatar of Pelor comes in, thinking he smelled Ambrosia. Wrong, but intrigued, the Sun God lingers for the banquet and blesses the house as he leaves. The King knights you the following day.")
Little flavorful rewards or punishments for success or failure is part of the fun of D&D in general, not just in combat.