Prepare the Baking Pan: Preheat the oven to 350°F (175°C). Line a 9 × 9-inch square baking pan with parchment paper, ensuring the pan is ready for the brownie batter.
Melt the Butter and Chocolate: Combine the unsalted butter and semi-sweet chocolate in a medium saucepan or microwave-safe bowl. If using a saucepan, warm the mixture over low heat, stirring with a wooden spoon or rubber spatula until the butter and chocolate melt together smoothly. Alternatively, microwave the mixture in 20-second intervals, stirring between each interval until fully melted and combined. Add the Sugars: While the melted chocolate mixture is still hot, stir in the granulated sugar and soft dark brown sugar until evenly incorporated.
Mix in the Eggs and Flavoring: Add the eggs to the chocolate mixture and beat until well combined. Stir in the salt and vanilla extract, mixing until evenly distributed.
Incorporate the Dry Ingredients: Add the cocoa powder and all-purpose flour to the mixture. Stir until everything is combined into a thick brownie batter, avoiding unnecessary mixing.
Soften the Cookie Butter: Transfer the cookie butter to a small microwave-safe bowl. Microwave for 20–30 seconds, or until it softens enough to become runny and easy to drizzle.
Fill the Baking Pan: Pour the brownie batter into the prepared pan and spread it evenly. Drizzle the softened cookie butter over the surface of the batter.
Create the Swirl Pattern: Use a knife to gently swirl the cookie butter through the brownie batter, creating a decorative marbled pattern. Avoid overmixing so the cookie butter remains visible in distinct swirls.
Bake the Brownies: Place the pan in the preheated oven and bake for approximately 30–35 minutes. The source lists a 30-minute cook time, so begin checking around that point. The brownies are ready when a knife inserted into the pan comes out clean, as specified in the original recipe.
Cool and Serve: Remove the brownies from the oven once baked. Allow them to cool before cutting into 16 small brownie bites or 9 larger brownies, as desired.
