Using a rubber spatula, stir about one-third of the egg whites into the batter to lighten the mixture. In two additions, gently fold in the remaining egg whites, making sure no white streaks remain but being careful not to deflate the whites. Equally divide the batter between the prepared pans. Bake side by side, rotating halfway through the baking time, until a toothpick inserted into the center comes out clean, about 40 to 45 minutes. Cool the pans on a rack for 10 minutes, then unmold the cakes onto the rack to cool completely. Wash the cake pans.