Preheat oven to 350°F. Grease a 9×13-inch baking dish with butter or shortening. Crust Layer: In a medium bowl (or directly in the prepared baking dish) combine the flour, powdered sugar, and melted butter. Stir with a fork until the mixture begins to come together, then use your hands to gently mix until a soft dough forms. Press the dough evenly into the bottom of the prepared baking dish. Bake for 12–15 minutes, until the edges are lightly browned, but the center still looks slightly underbaked. Do not overbake. The crust will continue to set as it cools. Let cool completely before adding the remaining layers