Preheat oven to 350°F . Line a 7" x 13" baking pan with aluminum foil. Grease pan and set aside.
Sift together flour, baking powder and salt. Set aside. Cream butter and sugar until light and fluffy. Add in yolks one at a time mixing well after each addition. Blend in dry ingredients in thirds alternately with milk in 2 additions. Set aside.
Beat egg whites and remaining sugar until stiff. Fold into the butter mixture. Transfer to prepared pan and top with chopped peaches. Bake for 10 minutes. Sprinkle streusel on top and bake for another 15 minutes or until done.
To make the streusel topping, cut in butter in flour, sugar and cinnamon using a pastry blender or 2 knives. Blend in cornflakes and use as required.