In a stand mixer fitted with the paddle attachment, combine the tapioca starch, cornstarch, sweet rice flour, millet flour, sea salt, xanthan gum and confectioners' sugar. Beat on low to combine. Add the butter and continue to mix on low until the mixture resembles coarse crumbs. Add the egg and mix on medium-high until the mixture just comes together.

Turn out onto a (rice) floured board and knead for several turns. Wrap in plastic and shape into a disk. Refrigerate for at least 2 hours.

Preheat the oven to 350 degrees. Remove the dough from the fridge and take off the plastic wrap. Dust a board (or the table or counter) with rice flour and roll dough out to between ¼ and ⅛ inch thick. Cut into desired shapes and place 1-inch apart on a cookie sheet that's been lined with parchment paper. Decorate with gluten free sprinkles, if desired.

Bake for 12-15 minutes, or until cookies are very lightly golden brown. Remove from oven and cool on a rack. Store in an airtight container.