For crust: Combine 1 1/2 c flour, powdered sugar and salt in a medium bowl. Cut in butter with a fork until mixtures resembles coarse crumbs. Press into a greased 9 x 9 baking pan in an even layer. Bake at 350 degrees for 15-18 minutes, until just golden at edges. To make filling: Whisk together eggs and sugar in a medium bowl until smooth. Stir in lemon juice, optional zest, then remaining flour. Pour filling over baked crust. Reduce heat to 325 degrees and bake 25-30 minutes, or until the filling has just set. Let cool for 30 minutes. Cut into 2-inch squares; sprinklw with powdered sugar. Makes 16.