Take butter out of the refrigerator and let soften slightly. Then cut into small chunks, about 1cm squares.
Put the flour, xantham gum, salt, and powdered sugar into a large bowl and mix well.
Add the butter chunks, and using a pastry cutter, incorporate the butter into the flours until the mixture just holds together (similar to a pie crust). If the crust is really not coming together, don't be afraid to add a couple TBs of cream or milk to help the process. Work the dough until there aren't any noticeable pieces of butter.
Use aluminum foil to cover the inside of a 9x9 inch pan, then grease with a small amount of butter or cooking spray. A neat trick to getting the aluminum foil into the right shape without ripping it is to put the foil around the outside of the pan first, to make the shape, then taking it off and placing the molded foil into the pan, pushing it into the corners.
Put the dough into the bottom of the pan, pressing it into the corners and making sure that it is even. It is okay if it begins to crumble, just push it in.
Bake at 350 degrees (F) for 15-20 minutes, or until very lightly brown.
While the crust is in the oven, in another mixing bowl add the sugar, lemon juice, cornstarch, lemon zest, and eggs. For this step, it helps to use an electric mixer so that there aren't any cornstarch clumps. Set this aside until the crust is done in the oven.
When the crust is done, pour the lemon/egg mixture over the top of the crust and place the pan back in the oven for about 30 minutes. Make sure to check on the bars as ovens cook differently. Rotate half way through if your oven tends to cook unevenly.
When the lemon curd has set, remove from the oven and place on a wire rack to cool. When the lemon bars are completely cool, lift the bars out by the aluminum foil.
Cut into 9, 12, or 16 pieces with a very sharp knife.