Heat the oven to 350°F (175°C). Grease a 9 x 13-inch baking pan or line it with parchment, leaving some overhang for easy lifting.

Beat the softened butter and 1/2 cup granulated sugar until combined. Mix in 2 cups all-purpose flour until a crumbly dough forms.

Press the dough evenly into the bottom of the prepared pan. Bake for 15 to 20 minutes, until the edges are just beginning to turn golden.

While the crust bakes, whisk together the eggs, 1 1/2 cups granulated sugar, 1/4 cup flour, and fresh lemon juice. Stir in the lemon zest if using.

Pour the lemon filling over the hot crust as soon as it comes out of the oven. Return the pan to the oven and bake for 20 to 25 minutes, until the center is set with only a slight wobble.

Cool the bars completely in the pan for at least 1 hour. For cleaner cuts, refrigerate for 1 to 2 hours, then dust with confectioners’ sugar and cut into squares.