Prep the pan: Heat oven to 350°F (175°C). Line an 8×8-inch (20×20 cm) pan with parchment, leaving a 1-inch overhang for easy lifting. Lightly grease the parchment.

Melt chocolate and butter: In a heatproof bowl, melt the butter and chopped chocolate together over a simmering pot of water, or microwave in 20–30 second bursts, stirring until smooth. Let cool 3–5 minutes so it’s warm, not hot.

Whisk in sugars: Stir in granulated and brown sugars until glossy and combined.

Add eggs and vanilla: Whisk in eggs one at a time until the batter looks thick and shiny. Mix in vanilla. Don’t overbeat—30 seconds is plenty.

Fold in dry ingredients: Sift or whisk flour, cocoa, and salt together. Fold into the wet mixture just until no dry streaks remain. If using chocolate chips, fold them in now.

Spread the batter: Pour batter into the prepared pan and smooth the top with a spatula.

Warm the swirls: Gently warm peanut butter and Nutella in separate small bowls for 10–15 seconds in the microwave to loosen. Dollop heaping teaspoons of each all over the surface.

Create the swirl: Use a butter knife or skewer to drag through the dollops in gentle figure-eights. Don’t over-swirl; 6–8 passes are enough for clean ribbons.

Bake: Bake 22–28 minutes, until the edges are set and the center still looks slightly soft. A toothpick should come out with moist crumbs, not wet batter.

Cool completely: Let brownies cool in the pan on a rack at least 1 hour. Lift out with parchment and slice with a warm, clean knife for neat edges.