Grease and flour a loaf pan. Cream butter, sugar, & egg yolks together. Beat for 1/2 hour!!!! Add flavoring and flour. Blend. In another bowl - beat egg whites until stiff. Fold gently into batter. Bake at 350 degrees for about 45 minutes - Then Increase heat to 375 for 15 minutes more. When toothpick comes out clean, remove from oven and cool. Turn out on plate, and allow to cool completely. This cake is fine served plain (topped with a spoonful of fruit or pudding, or may be glazed with your favorite glaze. The cake keeps well in a closed container. ***Notes - if you don't care for rum flavor, try brandy or sherry extract, or lemon, strawberry, maple, vanilla, etc. I have tried them all, and they are good. You can also slice it and make a torte by placing different jams or fillings between the layers.