It looks funnier out of context, but Bosco explained earlier that, when it comes to blood, vampires have tough dietary restrictions. And to be fair, he's controlling his blood lust to the best of his ability. He only burdens his girlfriend four times a year, and only for 200cc's at a time (less than a transfusion unit).
Bosco: "You can't imagine how difficult it is to find someone of this kind in a modern city like Tokyo. A girl who's a virgin, doesn't drink or smoke or do drugs? Why it's almost impossible!"
Nanami tries to bake a cake in Incident File: 03. But she gets the ingredients wrong and the oven explodes. Unfortunately, Nanami's a pyrokinetic who spooks easily. Her resultant panic blast scorches Ayaka and Mamoru, but her biggest concern?
Nanami: "It's all ruined. Maybe it was one tablespoon of baking soda, two cups flour."