Mix together flour, sugar, & butter with an electric mixer. Reserve 3/4 cup of this mixture for topping.
To remaining flour, sugar and butter mixture, add salt, baking powder, vanilla, milk, & egg yolks and lemon zest.
Beat egg whites until firm peaks form.
Gently fold the firm egg whites into the batter and pour into a greased 9" X 13" baking pan.
Sprinkle blueberries over the top of the batter, sprinkle the reserved 3/4 cup of flour/sugar/butter mixture on top of the blueberries.
Bake in 350 degree oven for 40-50 minutes.
If using the Lemon Glaze, pour over cake after it has cooled to warm.