Special equipment: 9 x 13 inch pan
For the crust: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch pan with butter and line with foil.
Cut together the butter, flour, granulated sugar and salt. Press into the prepared pan and bake until golden, about 20 minutes.
For the filling: Whisk together the granulated sugar, flour and eggs until smooth. Add the lemon zest and juice and mix until combined. Pour over the crust and bake until just set, about 20 minutes.
Allow to cool in the fridge for a minimum of 2 hours, then sift powdered sugar over the top before cutting into squares.