Preheat oven to 325 degrees Fahrenheit
Sift flour, baking powder, and salt together.
Beat egg whites until stiff and set to the side.
Using a stand mixer or hand beater, cream butter or coconut oil with maple syrup, aka whip 'em together till real good, for 4 to 5 minutes, until it's light and airy.
Add the egg yolk to the creamed butter mixture, beating well.
Mix in half of the dry ingredients, then half of the milk, followed by the rest of the dry ingredients and then the last of the milk.
Fold in egg whites, lemon juice, and the floured blueberries until just combined. Scoop batter into a 10 inch cast iron skillet.
Sprinkle the maple sugar over top of the batter or the mixed brown and white sugar.
Bake for 35 minutes.