Preheat oven to 350 degrees F. Spray a 9 x 18 x 1 inch jelly roll pan with cooking spray.
Combine flour, sugar and butter in a bowl with a pastry blender or pulse in a food processor until the butter is the size of small peas. Add water or milk and mix just until the dough holds together. Press into the pan in an even layer. Bake 15 minutes, until just beginning to brown on the edges.
While crust bakes, make the filling: In a medium bowl, mix sugar, flour, baking powder and salt with a fork. Beat in the eggs, juice and zest until smooth.
Spread over the warm crust and bake 15-20 minutes; the edges will be light brown and the top should look slightly dry. Remove to a cooling rack and cool 5 minutes. Use a sieve to shake powdered sugar over the entire surface to avoid any lumps.
Let cool 1 hour before cutting into squares. Store in a covered container at room temperature up to 2 days or in the refrigerator for longer storage.
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