Preheat the Oven: Preheat your oven to 350°F (175°C). Grease an 8×8-inch baking pan or line with parchment paper.

Melt Butter and Mix Sugar: In a medium saucepan, melt the butter over low heat. Remove from heat and stir in the granulated sugar until smooth.

Add Eggs and Vanilla: Beat in eggs one at a time, fully incorporating each before adding the next. Stir in vanilla extract.

Combine Dry Ingredients: In a separate bowl, whisk together cocoa powder, flour, salt, and baking powder.

Fold Dry Ingredients into Wet: Gently fold the dry ingredients into the butter mixture until fully combined. Fold in chocolate chips.

Prepare Peanut Butter Swirl: Pour the batter into the prepared baking pan and smooth the top with a spatula. Drop spoonfuls of slightly warmed peanut butter evenly over the top. Use a knife or skewer to swirl the peanut butter into the batter, creating a marbled effect.

Bake: Bake for 25–30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Avoid overbaking to maintain fudginess.

Cool and Slice: Allow brownies to cool completely in the pan on a wire rack. Lift out using parchment paper (if lined) and cut into 12 squares.