1. Preheat your oven to 325°.
2. Grease and line a 9×13 baking pan.  If your pan is metal, fully line it, bottom and sides with parchment paper.  Lemon reacts with uncoated metal and can leave a metallic taste in your bars.  For glass or ceramic baking pans you should be fine to just grease the pan, although a sheet of parchment in the bottom of the pan will always make it easier to remove bars after baking.
   
3. Zest 6 lemons.
   
4. Starting with your zested lemons, juice as many fresh lemons as it takes to get 1 full cup of fresh juice.  My lemons had especially thick rinds this time and I ended up using 8 to get enough juice.  Set the fresh juice aside for the filling.
5. In a food processor or blender, add the granulated sugar and the saved zest of 6 lemons. Pulse multiple times to incorporate lemon zest throughout the sugar.  This will make the pieces of zest smaller, as well as flavor and perfume the sugar.  Set the scented sugar aside while you make the crust.

Make the crust:

1. In a mixing bowl, combine 2 1/2 cups flour, powdered sugar, vanilla and melted butter. 
2. Stir and combine just until no dry ingredients are visible in the bowl.  The finished dough will be crumbly but moist and will form a ball if clenched in your fist.  This shortbread cookie dough is similar to my Orange Shortbread Cookies but without cornstarch. 
3. Press the crust into the bottom of the prepared 9×13 pan.  Use your fingers to spread the crust all the way to the edges in an even layer and gently press it together.
4. Bake at 325° for 17-18 minutes until the edges are barely turning light brown.
5. Mix the filling while the crust bakes.

Make the Filling:

1. Beat together the flavored sugar and flour in a mixing bowl with a hand mixer, in a stand mixer or in your food processor.
2. Add the eggs and lemon juice and beat until fully combined and the sugar has begun to dissolve in the eggs and lemon juice.  You should have a smooth, homogeneous mixture with all the ingredients fully incorporated and no sugar standing in the bottom of your bowl.
3. As soon as you remove the crust from the oven, use a fork to poke holes around the surface.
4. Pour the filling over the warm crust as soon as you remove it from the oven.
5. Place the lemon bars back in the oven and bake an additional 25 to 30 minutes or until the filling is just set in the center.
6. Cool the bars in the pan on a wire rack until they reach room temperature.
7. Refrigerate for at least 2 hours until cold and fully set.
8. Dust the tops of the lemon bars with a little extra confectioner’s sugar (powdered sugar) while they are still in the pan. I put a small amount of powdered sugar in a fine mesh strainer and gently shake it over the top of the full sheet of lemon bars.
9. Cut into bars and serve. The yield will be determined by the bar size you cut. A 4×5 or 4×6 grid is good for smaller, more square bars, while a 4×4 grid is easier to cut and yields a reasonable dessert portion of 16 bars.