1Preheat oven to 350°F.
2In a medium bowl or the bowl of a food processor, combine the flour, sugar and salt; add the butter and blend with a pastry cutter or fork, or pulse in the food processor until well blended and crumbly. Press into the bottom of an 8x8-inch pan and bake for 10 minutes, until barely golden around the edges.
3In the same bowl (no need to wash it), stir together the sugar, flour, baking powder and salt. Add the eggs, lemon zest and juice and stir until well blended and smooth. Pour over the base.
4Return to the oven for 25–30 minutes, until slightly golden on top and bubbly around the edges. Cool completely in the pan on a wire rack before cutting. (If you freeze them, they'll cut even more cleanly; just let them sit on the counter to thaw.) Sprinkle with icing sugar before or after you cut them.
5Makes 12 bars.