Preheat oven to 350 degrees. Spray a 9 inch cake pan with non stick spray
Separate the egg whites and yolks. Using a stand mixer or a hand held electric mixer, beat the egg whites at high speed until stiff peaks form. Pour the egg whites out into another bowm and set aside.
Cream the 1/2 cup butter with the sugar until blended. Add the egg yolks and the vanilla; beat well.
In a medium bowl, whisk together the flour with the baking powder and salt.
Add the flour alternately to the batter with the 1/2 cup of milk to the butter mixture. Then, by hand, fold in the egg whites into the batter. Set aside.
To make the blueberry topping: In a small saucepan, melt the 2 tablespoons butter with the brown sugar until the sugar is completely dissolved. Pour into prepared cake pan. Top with the blueberries.
Spread the cake batter on top of the blueberries evenly. The batter will be very thick, this is normal. Bake for 55-60 minutes until done. Toothpick check comes out clean.
Remove from oven, and cool the cake in the pan for 15 minutes, then invert the cake upside down onto a serving platter, but leave the cake pan on top of the cake for another 5-10 minutes to let the blueberries soak into the cake a bit. I like to tap the top of the cake pan a bit to help all of the topping release easier.