Mix flour and 1/2 C powdered sugar in bowl. Cut in 1 C butter till crumbly. Press into a 9x13 pan. Bake 20 minutes in preheated 350 degree oven.
Filling:
In mixing bowl, whisk together sugar, 4 beaten eggs, flour and baking powder, salt and lemon juice. Pour on top of the baked crust and bake another 25 minutes (or till set) at 350 degrees
Frosting:
Whisk together the butter, powdered sugar, milk and vanilla. Add lemon juice to thin as needed. Frost on cooled bars.