Hi Poppy, did you try to bake cake in Mircrowave. It does sometimes happens to with normal vanilla cake or even other cakes which I baked sometime ago. When baking cake in Microwave the top layer of the cake get completely exposed to heat while the inner part is still uncooked. So when you keep on baking till the inner part is cooked, the upper layer of the cake gets almost burnt. But anyway my family and I likes to eat, as it tastes better with the upper layer is crispy due to slightly burnt layer.