Combine 1 C flour, confectioners's sugar & shortening. Using a pastry blender or fork, combine until the mixture becomes a coarse meal. Press into bottom of an 11" x 7" greased baking dish. Bake for 20 minutes. Cool completely.
In a seperate bowl, combine the sugar, 2 tbsp flour, zest, baking powder, salt, eggs & lemon juice; whisk until well blended. Pour over cooled crust.
Bake an additonal 20 - 25 minutes. Cool on wire rack for 30 minutes. Cut into squares & dust with more confectioners' sugar.