Combine all ingredients in large mixing bowl. With your fingertips, rub together till the butter is pea sized. Alternatively, use a pastry cutter. Set aside.
- Preheat oven to 350 degrees. Grease and lightly flour a 9×13-inch baking dish.
- Sift together flour, baking powder, and salt.
- In stand mixer with paddle attachment, cream butter and sugar on medium-high for 2 minutes.
- With mixer on low, add milk and vanilla until combined.
- Add dry ingredients and mix until just combined. Don’t over-mix!
- In a metal bowl, beat the egg whites to stiff peak.
- Whisk ¼ of beaten egg whites into cake mixture to loosen the batter. Gently fold in remaining egg whites just until combined.
- Pour into greased and floured baking dish. Sprinkle crumb topping on top, gently pushing the crumbs into the batter every few inches.
- Bake for 40−45 minutes, rotating halfway through, or until a toothpick inserted in the