Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides.
Step 2: In a large saucepan, melt the unsalted butter over low heat. Remove from heat once melted.
Step 3: Add the granulated sugar to the melted butter and whisk until well combined.
Step 4: Whisk in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract. Whisk vigorously for about 1 minute.
Step 5: In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Do not overmix.
Step 7: Fold in the chocolate chips or chopped chocolate.
Step 8: Pour about two-thirds of the brownie batter into the prepared baking pan and spread evenly.
Step 9: Spoon dollops of the softened cookie butter over the brownie batter in the pan.
Step 10: Drop the remaining brownie batter in smaller spoonfuls over the cookie butter.
Step 11: Using a knife, skewer, or the back of a spoon, gently swirl the cookie butter and brownie batter together to create a marbled effect.
Step 12: Bake for 28-32 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not wet batter).
Step 13: Remove from the oven and let cool completely in the pan on a wire rack before lifting out and cutting into squares.