When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.) For the fluffiest frosting, be sure there is no sign of yolk in the egg whites; crack each egg white into a separate cup before adding that to the bowl with the rest. Also start with a clean, grease-free bowl; glass or metal is better than plastic. The cake layers can be made a day ahead. Cool completely and then wrap well in plastic wrap and foil. Store at room temperature.