1. Preheat oven to 350°. Line bottom and sides of a 13- x 9-inch pan with heavy-duty aluminum foil or parchment paper, allowing 2 to 3 inches to extend over sides; lightly grease foil.
2. Stir together 2 cups flour and 1/2 cup powdered sugar. Cut in butter using a pastry blender or fork until crumbly. Press mixture onto bottom of prepared pan.
3. Bake at 350° for 20 to 25 minutes or until lightly browned.
4. Meanwhile, whisk eggs in a large bowl until smooth; whisk in granulated sugar, lemon zest, and lemon juice. Stir together baking powder and remaining 1/4 cup flour; whisk into egg mixture. Pour mixture over hot baked crust.
5. Bake at 350° for 25 minutes or until filling is set. Let cool in pan on a wire rack 30 minutes. Lift from pan, using foil sides as handles. Cool completely on a wire rack (about 30 minutes). Remove foil, and cut into bars; sprinkle with powdered sugar.
To make ahead, prepare as directed. Cover tightly, and freeze up to 1 month.