Preheat oven to 180 degrees C. Grease a 20x30cm pan.
In a medium bowl stir together the 3 cups of flour with the icing 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.
I have made lemon bars for many years, but I've not ever made any just using egg whites, nor have I made any with a crust on the top. I probably might not again. I think I'm too ingrained to my old way of making them. And to boot, I don't think my lemons were very lemony--not a fault of this recipe. This all has to do with personal taste. Sorry about that! - 02 May 2013