Preheat the Oven: Heat the oven to 350°F (177°C). Grease a 5×5 inch baking dish and set it aside.

Make the Crust: In a medium bowl, beat the butter and sugar with an electric mixer until smooth. Add the flour, cornstarch, and salt and mix until a soft dough forms.

Bake the Crust: Press the dough evenly into the prepared baking dish. Bake for 15 to 18 minutes, until lightly golden. Remove from the oven.

Make the Filling: In a small bowl, whisk the egg, sugar, flour, and lemon juice until smooth.

Add the Filling: Pour the lemon mixture over the warm crust.

Bake the Bars: Bake for 15 to 17 minutes, until the edges are lightly browned and the center slightly jiggles when the pan is gently moved.

Cool and Slice: Let the bars cool completely in the pan on a wire rack, then cut into squares. Dust with powdered sugar if desired. Tip: For clean slices, dip a knife in warm water, wipe it dry, and cut. Repeat between slices.