1- Preheat oven to 350 degrees. Line 2 baking sheets with parchment.
2- In a food processor, grind the flours, pecans, sugar, salt, & xanthan gum together until the mixture resembles a coarse meal and no pecan chunks remain.
3- Add soft butter a bit at a time and pulse until mixture comes together and a dough forms. The dough will be quite soft.
4- Using your hands, form the dough into a wide, flat disk, picking up any loose pieces along the way to re-incorporate them.
5- Wrap disk tightly in plastic wrap and refrigerate until completely chilled, at least 2 hours, or overnight.
6- Remove dough from the fridge and allow to come to room temperature. Roll the dough into balls between your palms, about the size of 1 tablespoon. Place on lined baking sheet, spaced 1" apart.
7- Bake cookies 10-12 minutes until center is just set and edges are very light golden brown. The cookies will firm up as they cool.
8-Allow cookies to cool completely before covering in powdered sugar. Enjoy!