Melt the butter in a heavy-bottom medium saucepan over low heat. Once melted, pour into the bowl of a stand mixer.
In a medium bowl, sift together the rice flour, cornstarch, tapioca flour, xanthan gum, salt and baking soda. Set aside.
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.
Chill the dough in the refrigerator until firm, approximately 1 hour.
Preheat the oven to 375°F.
Shape the dough into 2-ounce balls and place on parchment-lined baking sheets, 6 cookies per sheet. Bake for 14 minutes, rotating the pans after 7 minutes for even baking. Remove from the oven and cool the cookies on the pans for 2 minutes. Move the cookies to a wire rack and cool completely. Store cooked cookies in an airtight container.