Loaded with fresh strawberries and a hint of lemon that act as a naturally tart counterpoint to the sweetness of mascarpone, the sponge cake has the butteriest, tenderest character. Air is held by butter, and butter traps that air when it is creamed. Then, why is my cake dense and not fluffy? The extra air in the batter will cause the cake to bake too dense, resulting in a deflated cake. Why is my sponge cake not light and fluffy?