Preheat oven to 350 F.
Beat egg whites until stiff peaks form and then set aside.
Cream butter and sugar.
Beat in egg yolks and vanilla.
In a separate bowl, mix flour, baking powder, and salt.
Add flour mixture alternatively with milk to sugar/butter mixture.
Fold in egg whites gently.
Add blueberries and stir in gently.
Pour mixture into 9" x 9" pan.
Mix together topping ingredients until crumbly and sprinkle over top of cake.
Bake for 40-50 minutes.