For the cake, blend flour, baking powder, and salt together in a mixing bowl.
In a separate bowl, beat egg whites and set aside.
In another bowl, cream butter and sugar. Then add half the flour mixture and half the milk and combine. Add the remaining half of the flour mixture and milk and combine. Don't overbeat.
Fold in beaten egg whites.
For the topping, combine all topping ingredients in a mixing bowl until crumbly.
Spread half of cake mixture in a greased 9 x 13 pan.
Sprinkle half of topping mixture over cake. Use a butter knife to gently swirl topping into mixture if.
Fold in remaining half of cake mixture. Sprinkle remaining topping mixture over top.
