Preheat oven to 325°F. Lightly grease a 9x13" baking pan with cooking spray or butter.
To make the crust: combine the flour, both sugars, lemon zest, salt and softened butter. You can either use a hand mixer or food processor to mix the dough. Mix until crumbly or until it resembles wet sand. Press dough onto the bottom of the pan. Bake for about 45 minutes or until crust is lightly golden around edges.
While the crust is baking, make the filling. In a large bowl, whisk together all of the ingredients.
Take the crust out of oven once it's done baking. Increase temperature to 350°F.
Carefully pour the lemon filling onto the crust and return pan back to oven. Bake for another 20 minutes or until filling is set.
Let cool thoroughly before cutting into squares. Chill in fridge to help them set up even faster. Dust with powdered sugar just before serving, if desired. Makes 24 squares.