Preheat & Prep. To start, preheat oven temperature to 350 degrees F. Line two baking sheets with parchment paper.
Combine Dry Ingredients. In a medium bowl, combine gluten-free flour, cocoa powder, baking soda, baking powder, and salt and whisk until evenly blended.
Cream Butter & Sugars. In the bowl of a stand mixer or a large mixing bowl, combine softened butter, granulated sugar, and brown sugar. Cream with a paddle attachment or hand mixer 2-3 minutes, or until light and fluffy.
Finish Wet Ingredients. Add the egg and mix until fully incorporated, then mix in milk, and vanilla. Scrape down the sides of the bowl.
Add Dry Ingredients and mix until no more dry patches remain.
Scoop & Shape Cookies. Use a small cookie scoop to measure 1 Tablespoon balls of dough. Roll each ball smooth between your hands then flatten halfway down with the bottom of a measuring cup, a piece of parchment, or your fingertips. (You should have 36 balls of cookie dough, which will make 18 sandwich cookies.)
Bake Cookies 6-7 minutes in preheated oven. For perfect cookies, use a cookie scoop to gently shape the cookies into perfect circles while they're warm. Remove from oven and let cool 1-2 minutes on sheet pan before transferring to a wire rack to finish cooling completely. Repeat, alternating baking sheets with each batch so you don't put cold dough on a hot baking sheet. Cool cookies completely before frosting.