Preparation

• Heat the oven to 400°F. Line a baking sheet with parchment paper.
• In a large bowl, whisk together the flour, cornstarch, brown sugar, baking powder, cinnamon, xanthan gum, baking soda, salt, nutmeg, and ginger until well combined. Add the butter pieces. Using a pastry blender, a fork, or two knives, cut in the butter until the mixture resembles coarse crumbs. Make a well in the center and set aside.
• In a small bowl, using a wire whisk, lightly beat the egg. Stir in the mashed sweet potatoes. Gradually whisk in the milk. Pour the liquid mixture into the well in the flour mixture all at once and stir with a fork just until the dry ingredients are moistened and the mixture comes together into a soft dough.
• Place the dough on a board or other surface lightly dusted with gluten-free all-purpose flour. With lightly floured hands, gently pat into a flat disk about 3⁄4 inch thick. Using a 2-inch biscuit cutter dipped in flour, cut out as many biscuits as possible. Gently gather the scraps into a ball, pat out, and cut out more biscuits. Place the biscuits about 1 inch apart on the prepared baking sheet. Brush the tops of the biscuits with melted butter, if desired.
• Bake until the biscuits are golden brown, 15 to 20 minutes. Immediately transfer to a wire cooling rack. Serve warm.