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 parchment-lined 9" x 13" baking pan. Bake 20 minutes, until crust is golden. (Well....) Remove from oven.
Prepare the filling. In a blender, puree the lemon, sugar, butter, vanilla and eggs until smooth, 2 to 3 minutes. Pour the filling into the pan.
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. Or so. I've increased the lemons, eggs, and sugar, so it may take longer now. 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.]
[May 2019. I added a little nutmeg and used a smaller pan to make them thicker. In a 9x13 pan, I can get 48 servings. Next time I may increase the spices to get a more flavorful crust.]
[Aug. 2019. I've increased the spices, lemon, vanilla, eggs, and sugar to make a bigger batch. This new recipe gets the name "Lemon Ginger Bars II." They're thicker than before, so I may need to bake them for 40 minutes next time. I'm also going to adjust the sugar and eggs.]