Tangy Lemon Squares

1. Prepare the Crust

Start by adjusting the oven rack to the middle position and preheating your oven to 350 degrees Fahrenheit. Line an 8-inch square baking pan with aluminum foil, creating a sling for easy removal later. In a medium bowl, whisk together the flour, granulated sugar, and salt. Add the melted butter and stir until the mixture is evenly combined. Press this crumbly mixture firmly into the bottom of the prepared pan, ensuring an even layer. Bake the crust until it turns a dark golden brown, which should take about 19 to 24 minutes.

2. Prepare the Lemon Filling

While the crust is baking, prepare the lemon filling. In a large bowl, whisk together the granulated sugar, flour, cream of tartar, and salt. Add the eggs and yolks, followed by the lemon zest and juice, whisking until smooth. Pour this mixture into a saucepan and cook over medium-low heat, stirring constantly, until the filling thickens and registers 160 degrees Fahrenheit on a thermometer. This should take about 5 to 8 minutes. Once thickened, remove the filling from heat and strain it through a fine-mesh strainer into another bowl to ensure a smooth texture. Stir in the butter pieces until melted and the mixture is silky.

3. Bake the Lemon Squares

When the crust is ready, pour the hot lemon filling over it, spreading it evenly. Tilt the pan gently to ensure even coverage. Bake the assembled squares for 8 to 12 minutes or until the filling is set and barely jiggles in the center.

4. Cool and Serve

Allow the lemon bars to cool completely in the pan, which will take at least 1.5 hours. Once cooled, use the foil sling to lift the bars out of the pan and transfer them to a cutting board. Cut into 12 even squares. If desired, dust with confectioners' sugar just before serving.