Natural Leavenings

The fat you used in your butter cake (butter or solid shortening) traps air bubbles, whether you have creamed it until light and fluffy (the creaming method) or stirred it with the other ingredients (the blending method) to form your batter. When you added eggs, you also created air bubbles. These two forms of leavening build natural aeration into your butter cake. Pound cakes, with their characteristically close, fine-grained, moist and dense textures, originated before chemical leavenings were heard of and rely exclusively on natural leavenings. In the oven, the liquid in the batter converts into steam through the air bubbles you have produced, expanding and leavening the cake.