Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan.
In a medium bowl, stir together the 3 cups of flour with the confectioners' sugar.
Cut in the butter with a fork or pastry cutter until the mixture resembles coarse crumbs. Press 1/2 of the mixture into the bottom of the prepared pan.
Bake for 15 minutes.
In a smaller bowl, stir together the egg whites and white sugar.
Add the flour and baking powder, mix until lumps are gone. Stir in the lemon juice.
Pour the lemon mixture over the baked crust, then sprinkle the remaining crust mixture evenly over the top.
Bake for 20 to 25 minutes in the preheated oven. Refrigerate to set before serving.
