Measure sifted flour, baking powder, and salt; sift together three times.
Cream butter or margarine. Gradually add remaining 1 cup sugar, and cream together until light and fluffy. Add sifted ingredients alternately with milk a small amount at a time, beating after each addition until smooth. Mix in flavorings.
In a mixing bowl, beat egg whites until foamy. Add 1/2 cup sugar gradually, and continue beating only until meringue will hold up in soft peaks.
Add meringue, and beat thoroughly into batter. Spread batter in a 15 x 10 x 1 inch pan which has been lined on the bottom with parchment paper.
Bake at 350 degrees F (175 degrees C) for 30 to 35 minutes. Cool cake in pan 10 minutes, then remove from pan and transfer to a wire rack to finish cooling. This cake may also be baked in two 9 inch round pans for 30 to 35 minutes, or in three 8 inch round pans for 25 to 30 minutes.