Step 1: Prepare the Brownie Batter

1. Preheat your oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.
2. In a medium bowl, whisk together the gluten-free flour, cocoa powder, baking powder, and salt.
3. In a separate microwave-safe bowl, melt the dark chocolate and butter together, stirring until smooth.
4. In a large mixing bowl, beat together the granulated sugar, brown sugar, eggs, and vanilla extract.
5. Slowly add the melted chocolate mixture to the egg mixture, stirring continuously.
6. Gradually fold in the dry ingredients until fully incorporated.

Step 2: Prepare the Cookie Dough

1. In another bowl, whisk together the gluten-free flour, baking soda, and salt.
2. In a separate bowl, cream the butter and sugars together until light and fluffy.
3. Add the eggs and vanilla extract, mixing until combined.
4. Gradually add the dry ingredients to the wet mixture, folding in the chocolate chips.

Step 3: Assemble and Bake

1. Pour the brownie batter into the prepared pan, spreading it evenly.
2. Drop spoonfuls of the cookie dough on top of the brownie layer.
3. Gently swirl the layers together with a knife for a marbled effect or leave them distinct.
4. Bake for 30-35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
5. Let cool completely before slicing and serving.