A small amount of simple syrup brushed onto butter cake layers keeps the cake fresh when served a day or two after baking. It can also be flavored with liqueur, citrus juice, or extracts. Poke the top of the cake layer all over with a thin skewer to enable the syrup to penetrate through the top crust, and then brush on the syrup. One tablespoon for the top and 1 tablespoon for the bottom of a 9 inch round cake layer is a good rule of thumb to moisten the cake without making it soggy or too sweet.