Whisk flour blend, baking soda, xanthan gum, and salt together in medium bowl; set aside. Whisk melted butter, brown sugar, and granulated sugar together in large bowl until well combined and smooth. Whisk in egg, milk, and vanilla and continue to whisk until smooth. Stir in flour mixture with rubber spatula and mix until soft, homogeneous dough forms. Fold in chocolate chips. Cover bowl with plastic wrap and let dough chill at least 30 minutes.
Adjust oven rack to middle position and heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Using 2 soupspoons (better yet, use cookie scoop) and working with about 1 1/2 tablespoons dough at a time, portion dough and space 2 inches apart on prepared sheets. Bake cookies, 1 sheet at a time, until golden brown and edges have begun to set but centers are still soft, 8-9 minutes, rotating sheet halfway through baking. Cookies will set upon cooling; don't overbake.
Let cookies cool on sheet for 5 minutes, then transfer to wire rack. Serve warm or at room temperature. (Cookies are best eaten on day they are baked, but they can be cooled and placed immediately in airtight container and stored at room temperature for up to 1 day.)
Notes
Be sure to use potato starch, not potato flour. Tapioca starch is also sold as tapioca flour; they are interchangeable. America's Test Kitchen strongly recommends We strongly recommend Bob’s Red Mill white and brown rice flours. For best results, weigh your ingredients. If you measure by volume, spoon each ingredient into the measuring cup (do not pack or tap) and scrape off the excess. Dough can be wrapped airtight and frozen for several weeks.