Cream together the softened butter, sugar, and salt until light and fluffy.

Add egg yolks one at a time, mixing well after each addition.

In a separate bowl, combine flour and baking powder.

Gradually add the dry ingredients to the wet ingredients, alternating with milk, and mix until just combined.

In another bowl, beat egg whites until soft peaks form.

Gently fold the beaten egg whites into the batter.

Lightly toss fresh blueberries with a small amount of flour to prevent them from sinking.

Gently fold the flour-covered blueberries into the batter.

Spread the batter evenly into a greased 9x13 inch baking pan.

In a separate bowl, mix together the remaining flour, sugar, and softened butter until the mixture resembles coarse crumbs.

Sprinkle the crumb topping evenly over the batter in the pan.

Bake in a preheated oven at 350°F (175°C) for 30 minutes, or until the cake is golden brown and a toothpick inserted into the center comes out clean.