Heat the oven to 350°F / 175°C. Line a 9 x 13-inch / 23 x 33-cm baking pan with parchment paper, leaving some overhang for lifting, or grease the pan thoroughly.

In a large bowl, whisk the melted butter and granulated sugar together until glossy and well combined. Add the eggs and vanilla extract, then whisk until the mixture looks smooth.

Add the all-purpose flour, unsweetened cocoa powder, and salt. Stir only until no dry streaks remain, taking care not to overmix the batter.

Spread the brownie batter evenly in the prepared pan. Smooth the surface with a spatula so the peanut butter and Nutella can be distributed across the whole pan.

Warm the peanut butter and Nutella separately in the microwave for about 15 to 30 seconds each, just until slightly fluid. Do not make either spread hot or runny.

Drop spoonfuls of the warmed peanut butter and Nutella over the brownie batter. Run a knife gently through the toppings in a few strokes to create distinct ribbons without fully mixing them into the batter.

Bake for 25 to 30 minutes, until the edges look set and a toothpick inserted near the center comes out with moist crumbs rather than wet batter. Avoid waiting for a completely clean toothpick.

Cool the brownies in the pan for at least 30 minutes before slicing. For cleaner squares, chill the cooled pan briefly, then lift the brownies out using the parchment and cut with a sharp knife.