Set the oven temperature to 350°F (175°C). With an overhang for convenient removal later, line the baking dish with parchment paper.
For the crust, put the flour and powdered sugar in a bowl. Using a pastry cutter or fork, cut in the cooled butter until the mixture is the consistency of coarse crumbs.
Evenly press the crust mixture into the baking dish's bottom. Bake until gently browned, 15 to 20 minutes.
As the crust bakes, get the filling ready. Mix the flour and granulated sugar in a separate basin.
Whisk in the eggs, zest, and lemon juice until smooth.After the crust has baked, pour the filling over it and put the dish back in the oven.
Bake for a further twenty to twenty-five minutes, or until the sides are gently brown and the filling is set.
Take out of the oven and allow it to cool fully in the pan. After cooling, cut into bars and, if preferred, dust with powdered sugar.