Any of the cupcakes from Starring Rolls at HS are really good- and HUGE! The tie-dye cheesecake at Pop Century is really good- even if your not staying there, if you have time, its worth going over there just for that! Its Rainbow colored cheesecake on top with red velvet cake on the bottom.
As a HUGE dessert lover myself, here is my Disney tradition: each morning while eating breakfast at my resort, I'll go ahead and buy a dessert..then when I am done eating, I bring it back to my room and store it in the fridge...then when I get back from the parks at night around midnight, I have this delightful delicious morsel waiting for me! You dont have to worry about the food place being closed or out of the dessert you want this way either!