Preheat oven to 350°F.

For the crust: Cream the butter and sugar and lemon zest until light. Combine the flour and salt and add to the butter until just mixed. Dump the dough into prepared* baking dish, flatten the dough with floured hands and press it into an 8- by 10- by 2-inch or 9- by 9- by 2-inch baking dish. Chill for about 15–20 minutes.

Bake the crust for 15 to 20 minutes, until very lightly browned. Let cool on a wire rack. Leave the oven on.

For the filling: Whisk together the eggs, sugar, lemon zest, lemon juice, and flour. Pour over the crust and bake for 30 to 35 minutes, until the filling is set. Let cool to room temperature. Then refrigerate.

Cut into squares and dust with powdered sugar.

* Preparing pan with non-stick foil or parchment works great.