Preheat oven to 350 degrees.
Cut together flour, sugar and butter in a large bowl using a butter dough blender until combined and no pieces bigger than 1/4 inch remain. (Reserve 1/2 cup of this mixture for the topping before next step.)
Add salt, baking powder, vanilla, milk & egg yolks and blend well by hand.
Beat egg whites until firm, and then fold gently into the batter until well combined. (Don't overmix, or your cake will not be fluffy.)
Pour batter into a greased 9"x13" baking pan.
Sprinkle blueberries evenly on top, and then remainder of 1/2 cup reserved mixture from step 1.
Bake in 350 degree oven for 40-50 minutes, or until toothpick comes out batter-free and top and edges are golden.