Preheat the oven to 325°F. Line rimmed baking sheets with unbleached parchment paper and set them aside.

In the bowl of your stand mixer fitted with the paddle attachment, or the bowl of a food processor fitted with the steel blade, add the flour, xanthan gum, baking powder, baking soda, salt, and brown sugar.

Whisk with a separate handheld whisk or pulse to combine. Add the butter, egg, and vanilla, and beat or process until well-combined. Add the milk, and beat or process until the dough clumps.

Either scoop dough by the level tablespoonful, or pipe an equivalent amount through a pastry bag fitted with a plain piping tip (for more consistent cookie sizing) about 1 inch apart on the prepared baking sheets.

The dough will spread during baking, but not a lot. If you have piped the dough, with wet fingers, gently flatten the top of each raw cookie if anything is sticking up.

Place the baking sheets in the center of your preheated oven and bake for 15 to 17 minutes, or until the cookies are uniformly golden brown in color.

Allow the cookies to cool completely on the baking sheets. Store any leftover cookies, completely cooled, in a sealed glass container at room temperature to preserve crispness.