Preheat oven to 375ºF. Grease a standard 12 muffin pan.
In a large mixing bowl, place flour, baking powder, salt, xanthan gum and baking soda; blend well. Using a small holed grater, grate 1 stick of very cold butter into flour mixture. Blend together flour and butter using a pastry blender or two forks until the mixture resembles coarse meal.
In a large measuring cup, mix milk, vinegar and beaten egg together. Pour milk mixture into the center of the dry ingredients and mix with a large spoon. Batter will be moist and very thick. Evenly spoon batter into the prepared pan and using your fingers dipped in milk, lightly flatten the tops of the biscuits.
Bake biscuits for 22-25 minutes or until lightly golden brown. Remove biscuits from pan and serve warm or cool on a wire rack.