To Make the Streusel:

1. Whisk together the flour, brown sugar, cinnamon, and salt in a medium-sized bowl. 
2. Mix in the butter with a sturdy spatula or spoon until completely combined and a soft, thick crumble forms. Set aside. 

To Make the Cake:

1. Preheat the oven to 350°F. Grease an 8-by-8-inch pan with cooking spray and set aside.
2. In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on high speed until light and fluffy, about five to seven minutes.
3. As the butter and sugar are creaming, whisk together the flour, baking powder, and salt in a separate medium-sized bowl. 
4. Whisk together the whole milk and buttermilk in a separate small bowl. 
5. Whip the egg whites with a whisk in a separate medium-sized bowl until very soft peaks form.
6. Add the flour mixture and milk alternately to the butter-sugar mixture at low speed until combined, starting and ending with the flour. Mix until a smooth and thick batter forms, about one minute. Don’t overbeat. 
7. Remove the bowl from the stand mixer and fold in the beaten egg whites with a spatula. Continue folding until no streaks of beaten egg whites remain. 
8. Fold in the blueberries until evenly distributed in the batter. 
9. Spread the batter into the prepared pan. Evenly crumble the reserved streusel on top of the batter.
10. Bake for 50-55 minutes, until the cake is an even golden-brown color on the surface and a toothpick inserted into the center comes out clean. 
11. Allow to cool slightly to serve warm, or allow to cool completely in the pan to serve at room temperature.