1) Melt the butter in a heavy-bottom medium saucepan over low heat. Once melted, pour into the bowl of a stand mixer.
2) In a medium bowl, sift together the rice flour, cornflour, tapioca flour, xanthan gum, salt and bicarbonate of soda. Set aside.
3) Add both of the sugars to the bowl with the butter and using the paddle attachment, cream together on medium speed for 1 minute. Add the whole egg, egg yolk, milk and vanilla extract and mix until well combined. Slowly incorporate the flour mixture until thoroughly combined. Add the chocolate chips and stir to combine.
4) Chill the dough in the refrigerator until firm, approximately 1 hour. Shape the dough into 60g balls and place on greaseproof paper-lined baking trays, 6 biscuits per sheet. Bake for 14 minutes, rotating the trays after 7 minutes for even baking.
5) Remove from the oven and cool the biscuits on the trays for 2 minutes. Move the biscuits to a wire rack and cool completely. Store in an airtight container.