Line a couple of cookie sheets with baking parchment paper.
In a large mixing bowl measure out the gluten-free flour, oats, baking powder, baking soda, xanthan gum, and salt and mix until all combined.
In another bowl mix together the oil, milk, egg and vanilla essence.
Add the wet ingredients to the flour ingredients and mix them together. There is no need to cream or overmix the dough.
Add the chocolate chips and carefully stir them in. You should have a soft (but not runny) dough.
Using a desert spoon (or a large tablespoon), scoop the dough and drop on your baking trays, making sure that you leave good space between each cookie. Use more trays if needed as the cookies will spread a little.
Place in the middle of your oven and bake for 10 minutes or until the biscuits are nicely golden brown.
Using the greaseproof paper, lift your cookies from the baking tray and place them on the cooling rack. Leave to cool down completely.