Bake in a preheated 350 degree oven
1. Beat the butter and sugar in a large mixing bowl for about 5 minutes, until light and fluffy.
2. Sift together flour, baking powder and salt and set aside.
3.Whisk the egg whites, milk and vanilla extract together until well blended.
4. Add the flour alternately with the egg mixture while beating with an electric mixer. Always start with flour and end with flour. Add 1/3 of the flour to the butter mixture and beat until incorporated, add egg mixture, beat until incorporated. Repeat with flour, then egg mixture and lastly flour. Only beating until just incorporated. Scraping bowl often.
5. Pour batter into prepared 9x13 inch greased baking pan, smoothing batter to edges of pan. Bake 25 to 30 minutes testing with a toothpick for doneness.
6. Cool cake completely. With the handle of a wooden spoon poke holes in the cake.