Preheat the oven to 375 degrees F (190 degrees C). Grease two 8-inch round cake pans.
Cream butter and sugar until light and fluffy. Sift flour with baking powder and salt, and add to creamed mixture alternately with milk. Beat well. Stir in vanilla.
In a large glass or metal mixing bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Divide batter into prepared pans.
Bake in preheated oven until cake springs back when lightly touched with a finger and a toothpick inserted in the center comes out clean, about 25 minutes.
Allow to cool in pans for 10 minutes, then invert onto a wire rack to cool completely. Cover with white butter icing, and decorate as desired.