Low-fat lemon squares

So, now that I have to eat low-fat food, I have had to come up with recipes for treats. Some things have been more successful than others. The lemon squares turned out quite well, so I'm posting the recipe for them. The crust is not quite as crunchy as when you use butter, but it's still good.

1/3 c. low-fat margarine (like Brummel and Brown)
1/4 c. sugar
1 c. flour
4 egg whites
3/4 c. sugar
2 T. flour
2 t. shredded lemon peel (opt.)
3 T. lemon juice
1/4 t. baking powder
2 drops yellow food coloring (optional, but it looks weird without it)
powdered sugar

Mix margarine and 1/4 c. sugar together. Add the 1 c. flour and combine until crumbly. Press into bottom of an 8x8 pan. Bake at 350 for 15 to 18 minutes.
Meanwhile, for the filling combine egg whites, 3/4 c. sugar, 2 T. flour, lemon peel, lemon juice, baking powder and food coloring.
Pour filling over crust once it is baked and bake 20 minutes more or until center is set. Cool and dust with powdered sugar.