
I always substitute limes for lemons — the taste and color is more interesting, and somehow it just seems cooler to use lime juice.
Preheat oven to 375 degrees. Grease an 8-inch square baking pan. Blend 1 cup flour, butter, 2 tablespoons sugar, 1 tablespoon lime zest, and salt with a pastry blender until mixture forms a loose paste. Turn out into the baking pan and pat evenly onto the bottom, forming a 1/2-inch edge along the pan sides. Bake until the crust is pale golden, 15 to 20 minutes. Remove from oven, then reduce the temperature to 350 degrees. Cool crust 10 minutes.
While the crust cools, whisk together remaining flour, sugar, and lime zest along with lime juice, eggs, and baking powder. Pour the filling onto the partially cooked crust, then bake until filling is set, 25 minutes. Cool completely, then chill at least 2 hours before cutting into 16 squares. Sprinkle with confectioners' sugar. To freeze, wrap individual squares in plastic wrap and store in an airtight container. (Don't worry about defrosting these completely; lots of folks think they taste even better frozen.)
What's Good for YouJustification for eating half these bars yourself: Citrus zest is packed with limonene, an antioxidant that reduces the risk of skin cancers.