Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9x13 inch pan or three 8 or 9 inch round layer cake pans.
Cream the butter and sugar together until smooth and fluffy. Mix in the vanilla extract.
Separate the eggs and set the egg whites aside. Add the egg yolks one at a time to the creamed mixture, beating after each one.
Measure the baking powder into the flour, sift a little of the flour into the creamed mixture. Continue adding the flour and the milk alternately, while continually beating the mixture. Beat the cake batter until a few air bubbles show.
Beat the 4 egg whites with a dash of salt until stiff. Gently fold egg whites into the batter. Pour batter into prepared pan(s).
Bake at 350 degrees F (175 degrees C) until cake(s) test done, approximately 25 to 35 minutes.