Preheat oven to 350°. In a large mixing bowl, combine the 21/2 cups flour and 3 tablespoons sugar. Using a pastry blender or your fingers, cut or rub in butter until mixture is crumbly.
Press into the bottom of a greased 15x10x1-inch baking pan. Bake for 15 minutes or until golden.
Meanwhile, in a medium bowl, stir together eggs, 21/2 cups sugar, 1/4 cup flour, the lemon zest, lemon juice and baking powder until combined.
Pour filling over the hot crust. Bake bars for about 20 minutes more or until lightly browned around the edges and the center is set. Cool in pan on wire rack. Sift with powdered sugar. Store in refrigerator.