Pudding cakes are little culinary marvels. Take what is basically a thin cake batter, fold in whipped egg whites, put the batter into a baking dish (or several ramekins as I do here), and voilà! As it bakes, the batter separates and forms a sponge cake on the top with a thick, kind of custardy pudding on the bottom. Lemon pudding cakes are common in the South, but I've swapped lemon for Key lime juice.