This has been my go-to lemonsquare recipe for years.
1/2 cup (1 stick) unsalted butter, room temperature
1/4 cup powdered sugar
1 cup all purpose flour
1/8 teaspoon salt
1 cup granulated white sugar
2 large eggs
1/3 cup fresh lemon juice (approximately two large lemons)
1 Tablespoon (5 grams) grated lemon zest
1 teaspoon lemon extract (optional)
2 tablespoons all purpose flour
Preheat oven to 350 degrees F (177 C) and place the oven rack in the center of the oven. Butter, or spray with a non stick vegetable spray, an 8 x 8 inch (20 x 20 cm) baking pan.
Shortbread Crust: In the bowl of your electric mixer, or with a hand mixer, beat the butter and sugar until creamy and smooth. Add the flour and salt and beat until the dough just starts to come together. Press onto the bottom of your prepared pan and bake for about 18 -20 minutes, or just until lightly brown around the edges. Remove from oven and place on a wire rack to cool while you make the filling.
Lemon Filling:In your electric mixer, or with a hand mixer, beat the sugar and eggs until nice and smooth. Add the lemon juice and zest and stir to combine. Fold in the flour. Pour the filling over the shortbread crust and bake for about 17-20 minutes, or just until the filling has set. Remove from oven and place on a wire rack to cool. After cooled, cut into squares and dust with powdered sugar, if desired (I don’t desire.)
From The Joy of Baking