Beat egg whites until stiff peaks form.
Cream butter and sugar until light and fluffy.
Beat in dry ingredients, and buttermilk, alternating between the two. Add vanilla.
Slowly fold in egg whites.
To make crumble topping, combine ingredients thoroughly with your hands.
Pour batter into a greased 9x13 pan.
Spread crumble topping evenly over it, and bake for 45 min at 350 degrees F. Until no longer jiggly!:)
I've made this too many times. So so so delicious! Perfect bring to work snack!