Preheat the oven to 350 degrees F.
In a stand mixer bowl or a food processor, combine the 6 1/2 tablespoons butter cut into pieces, 3/4 cup of brown rice flour, the arrowroot flour, 3 tablespoons of cane sugar, salt, and xanthan gum. Mix or pulse until small crumbs remain.
Line an 8" square pan with parchment. Press the crust mixture into the lined pan, with the back of a spoon or rubber spatula to evenly get into the crust the corners.
Bake the crust for 15-18 minutes or until the edges are beginning to turn golden brown.
Make the filling in the meantime, in the stand mixer bowl or using a medium bowl and a hand mixer. Add the eggs, 1 1/4 cups sugar, 3 tablespoons of brown rice flour, lemon juice, lemon zest, cornstarch, and 1 1/2 tablespoons of butter. Using the whisk attachment blend until well mixed and slightly frothy.
After the crust is finished with it's first bake, pour the filling in and return to the oven for 18-25 minutes or until the edges are browned and the top is a little golden.
Remove to a wire rack to cool completely (for 3 to 4 hours is best). Dust with powdered sugar. Cut into squares and enjoy!