This recipe is from Bonnie Stern's 'Essentials of Home Cooking:'
Pastry
1 cup all-purpose flour
1/3 cup brown sugar
1/2 cup butter, cut in cubes
Filling
2 eggs
1 cup granulated sugar
2 Tbsp all-purpose flour
1/2 tsp baking powder
1/4 cup lemon juice
2 Tbsp lemon peel
2 Tbsp icing sugar for topping
1) Mix the crust ingredients, cutting the butter in as you would for a pie dough.
2) Press it into an 8-inch square pan that has been lined with parchment paper. Bake for 20 to 25 minutes at 350F until lightly golden.
3) Meanwhile, make the filling: beat eggs and granulated sugar. Stir in the flour, baking powder, lemon juice and peel. Pour over base, and bake for 20 to 25 minutes until set and brown around the edges. Cool.
4) Dust with icing sugar. Cut into squares.
I added a bit of vanilla and didn't have 2 Tbsp worth of lemon peels, so the result was relatively mellow for a lemon square... actually I liked it better than the regular super-lemony, tart lemon square.