Sift flour, baking powder, and salt together 3 times.
Beat egg whites until stiff and set aside.
Cream butter.
Mix sugar into butter.
Add egg yolks to the butter and sugar, beating well.
Add in dry ingredients, alternating with the milk.
Fold in egg whites, lemon juice, and the floured blueberries.
After scooping batter into a greased 10" cast iron pan, sprinkle top of batter with a 1/4 cup of mixed white and brown sugar.
Bake at 350 for 30 minutes.