In a medium mixing bowl, whisk together the gluten-free flour, cocoa powder, baking soda, and salt. Whisking helps evenly distribute the cocoa and leavening so the cookies bake consistently. Set the bowl aside.

In the bowl of a stand mixer fitted with the paddle attachment or whisk attachment, combine the granulated sugar, brown sugar, and softened butter. Beat on medium speed for about 2–3 minutes, until the mixture becomes light, fluffy, and slightly pale in color. Properly creaming the butter and sugars helps create a smooth cookie dough and improves the cookie texture.

Add the egg and vanilla extract to the butter mixture. Mix on medium speed until fully incorporated, scraping down the sides of the bowl as needed to ensure everything is evenly combined.

Gradually add the flour mixture to the wet ingredients. Mix on low speed just until the dough comes together. Avoid overmixing, as this can make gluten-free cookies dense.

Using a ½ tablespoon (0.5 oz) cookie scoop, portion the dough into small balls. This ensures the cookies are uniform in size so they bake evenly.

Place the cookie dough balls onto a parchment-lined baking sheet and refrigerate for about 10 minutes. Chilling helps firm up the dough, making it easier to flatten and helping the cookies maintain their shape while baking.

While the dough chills, preheat your oven to 350°F and line another baking sheet with parchment paper.

Place a piece of parchment paper over the chilled dough balls. Using the bottom of a mason jar or flat glass, gently press each dough ball into a thin cookie disc, about ¼ inch thick. Oreo-style cookies are thin, so flattening them well is important.

Transfer the flattened cookies to the prepared baking sheet, spacing them about 2 inches apart to allow for slight spreading during baking.

Bake cookies for 8–10 minutes, or until the cookies are set around the edges. The centers may still appear slightly soft, but they will firm up as they cool.

Allow the cookies to cool on the baking sheet for about 3 minutes before transferring them to a wire cooling rack. Let them cool completely on the wire rack before adding the filling.

In a mixing bowl, whisk or beat together the softened butter, powdered sugar, and vanilla extract until smooth, creamy, and fluffy. If needed, beat for 2–3 minutes to create a light texture similar to classic Oreo filling.

Once the cookies have completely cooled, pipe with a piping bag or spread with offset spatula the filling onto the flat side of one cookie. Place another cookie on top and gently press down to create a sandwich cookie.