Preheat the oven to 350 °F.

In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy. Then combine the flour and salt and, with the mixer on low, add to the butter. Blend until just combined.

Gather the dough into a ball and flatten it with floured hands and press it into a well oiled 9 by 13 inch baking pan.

Bake the crust for 15 to 20 minutes, the crust will still be very pale to very lightly browned at this point. Let cool on a wire rack.

Meanwhile, whisk together the eggs, sugar, lemon zest, lemon juice, and flour. Pour the filling over the crust and then bake for 30 to 35 minutes, or until the filling is set. Let cool to room temperature before removing from the pan.

Cut into small squares or triangles and dust with confectioners' sugar before serving.