Preheat oven to 200ºC .
Butter an 8-inch square pan, or use baking paper.
Combine butter, confectioners sugar, 1 cup flour and salt and press into the bottom of the prepared pan, making a crust.
Combine eggs, granulated sugar, 2 1/2 tablespoons flour, lemon juice and lemon zest and beat until light in colour. Use the juice of about 1/2 a lemon.
Pour over crust and bake for 25 – 30 minutes. Let cool until cool to the touch and dust top with confectioners sugar – cut into uniform squares.