Preheat oven to 350 F. Mix together flour, confectioners' sugar, and salt, and everything but the butter. Cut in butter or use food processor with quick on/off movements. Pat down well into ungreased 18" x 13" baking pan. Bake 20 minutes, until crust is golden. (Well....) Remove from oven.
Prepare the filling #1. In a blender, puree the lemon, sugar, butter, vanilla and eggs until smooth, 2 to 3 minutes. This makes about 3 cups filling. Pour the filling into the pan. Do it again wih filling #2.
Bake until puffed and golden and the filling jiggles only slightly when tapped, for 20 minutes. Turn the pan around and bake it another 13-15 minutes. The filling will brown around the edges, but should not be overly brown in the center; if it begins to brown too quickly, cover the top lightly with aluminum foil and continue baking. Remove and cool on a rack. Sprinkle heavily with powdered sugar.
[Sep. 2017. I combined three other recipes to make this.]
[Jan. 2018. I used Bob's GF flour and a little xanthan gum to make a GF batch.]