Step 1: Preheat the Oven
• Preheat Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Step 2: Prepare the Brownie Layer
• Mix Ingredients: In a medium bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until smooth.
• Add Dry Ingredients: Stir in the cocoa powder, flour, salt, and baking powder until just combined. Set aside.

Step 3: Prepare the Peanut Butter Layer
• Cream Peanut Butter: In another bowl, beat together the peanut butter, softened butter, granulated sugar, and brown sugar until creamy.
• Add Egg and Vanilla: Mix in the egg and vanilla extract until well combined.
• Add Dry Ingredients: Gradually add the flour, baking soda, and salt, mixing until just combined.

Step 4: Swirl the Doughs
• Combine Doughs: Drop spoonfuls of the brownie batter and peanut butter dough onto the prepared baking sheet, alternating between the two.
• Swirl: Use a toothpick or the tip of a knife to gently swirl the two batters together, creating a marbled effect.

Step 5: Bake the Cookies
• Bake: Bake in the preheated oven for 10-12 minutes, or until the edges are set and the centers look slightly underbaked.
• Cool: Remove from the oven and let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.