Preheat the oven to 190 C / Gas 5. Mix together the flour, xanthan gum, bicarb and salt, and set aside.
Cream the butter with the caster and brown sugars. Beat in the egg and vanilla. Gradually mix in the dry ingredients, then stir in the chocolate chips. Drop by teaspoonfuls onto a greased or lined baking tray. Dip spoon in water and then lightly flatten each cookie with the back of the spoon.
Bake the cookies for 9 to 11 minutes, or till golden brown. Remove from oven and let cool on the tray for 2 minutes before transferring to a wire rack to cool completely.