Preheat oven to 350 degrees F.
Sift flour with baking powder and salt.
Beat egg whites until foamy. Gradually add 1/2 cup sugar, beating thoroughly after each addition. Continue beating until mixture forms soft peaks.
Cream butter; gradually add 1 cup sugar; cream until light and fluffy.
Alternately add flour mixture and milk, small amount at a time; beating after each addition until smooth.
Mix in flavoring, fold in egg whites.
Pour in two 9-inch round cake pans that have been lined on bottom with wax paper. Bake for about 30 minutes, or until top springs back when pressed lightly. Cool.