Step 1: Prepare the Brownie Dough

1. In a medium bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. The mixture should look glossy and well-combined.
2. Add the egg and vanilla extract, whisking until fully incorporated.
3. Sift in the cocoa powder, baking soda, and salt. Stir until the dry ingredients are evenly distributed.
4. Gradually mix in the flour until the dough comes together. The dough will be slightly sticky but manageable.
5. Cover the bowl with plastic wrap and chill in the fridge for 20 minutes. Chilling helps firm up the dough, making it easier to handle.

Step 2: Make the Peanut Butter Dough

1. In a separate bowl, beat the creamy peanut butter, softened butter, granulated sugar, and brown sugar until light and fluffy. This step is key to achieving that tender, melt-in-your-mouth texture.
2. Add the egg and vanilla extract, mixing until smooth.
3. Stir in the baking soda, salt, and flour until the dough is cohesive.
4. Like the brownie dough, chill this mixture for 20 minutes to firm it up.

Step 3: Shape and Swirl

1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Scoop 1 tablespoon of the brownie dough and 1 tablespoon of the peanut butter dough. Gently roll them together to create a marbled ball. Be careful not to over-mix, as you want to see distinct swirls of chocolate and peanut butter.
3. Place the dough balls on the prepared baking sheet, spacing them about 2 inches apart. Lightly press down on each ball to flatten it slightly.

Step 4: Bake to Perfection

1. Bake in the preheated oven for 10-12 minutes, or until the edges are set but the centers remain soft. This ensures the cookies stay fudgy and chewy.
2. Remove the baking sheet from the oven and let the cookies cool for 5 minutes before transferring them to a wire rack. This brief resting period helps the cookies hold their shape.