Preheat the oven to 350°F. Lightly grease an 8" square pan or 9" round pan., To make the crust: Combine the dry ingredients in a small bowl, whisking to blend. Add the cold butter, working it in with your fingers or a pastry blender until the mixture is evenly combined and crumbly., Dump the mixture into the prepared pan, shake to distribute, and press the crust into the bottom and about 1/2" up the sides of the pan., Bake the crust until it's light golden brown, about 8 to 10 minutes., To make the filling: While the crust is baking, whisk together the filling ingredients., Remove the crust from the oven, and pour the filling over the hot crust. Return the squares to the oven and bake them for 14 to 18 minutes, until the filling appears set., Remove the squares from the oven, and allow them to cool in the pan before cutting into 2" pieces. Cover the squares, and refrigerate until ready to serve.