Whisk together flour, baking powder and salt in a small bowl and set aside.
In an electric mixer cream butter with the sugar until smooth. Add the egg whites one at a time and mix until combined, then add the vanilla.
Alternately add the flour and water a little at a time to the sugar mixture until both the flour and water are all added and no dry spots remain. Pour in chocolate chips and mix just until the chips are evenly distributed in the batter.
Pour batter into a greased 9x13" pan. Bake for 20-25 minutes or until the middle is set and the top is light golden brown.
Let cool slightly before serving. Cake is best when served just a little bit warm.