Preheat oven to 350 degrees. Sift together flour, baking powder, and salt. Beat egg whites and set aside.
Cream butter and sugar. Add flour mixture and milk alternately until combined. Don't overbeat. Fold in beaten egg whites with a rubber spatula. Spread in a well-greased cake pan.
In a separate bowl, combine topping ingredients with a pastry cutter until crumbly. Sprinkle all over the top.
Bake for 30 to 35 minutes, or until no longer jiggly. Serve warm---delicious!