Make the crust: In a food processor, pulse together the powdered sugar, butter, vanilla, and salt. Add the egg and pulse until just combined. Add all the flours and the xanthan gum, then pulse until just combined. Scrape down the sides of the bowl and pulse again, until just combined (don’t overmix). Flatten the dough into a 1-inch disk, wrap in plastic wrap, and refrigerate for 1/2 hour.

Roll the dough between two pieces of plastic wrap into a 1/4-inch-thick circle. Place the dough into a 9-inch tart pan with a removable bottom, pressing it lightly into the sides of the pan, then trimming any overhanging dough with a knife. Prick the dough with a fork in 20 places, then freeze the dough for 1 hour.

Preheat the oven to 325°F and bake crust for 25 minutes, or until golden brown. Set aside.

Make the lemon curd: Lower the oven temperature to 300°F. In a saucepan, combine the sugar, lemon zest, and lemon juice. Vigorously whisk in the eggs. Warm the mixture over medium-low heat and cook, whisking constantly, until the mixture starts to thicken, 5 to 8 minutes. Remove the pan from the heat and add the cubed butter, a few pieces at a time, whisking to melt, until all the butter is melted. For the smoothest curd, strain the mixture through a fine mesh sieve into a bowl.

Finish the tart: Pour the lemon curd into the tart crust and bake until the curd is set but still jiggles slightly, 15 to 18 minutes. Let cool completely, then remove the tart from the pan and serve.