Preheat and prepare: Set your oven to 180°C (fan 160°C). Line a 20cm square baking tray with parchment paper, leaving some overhang for easy lifting later. The paper should feel crisp and snug against the edges.

Mix the wet ingredients: In a large bowl, whisk the melted butter and caster sugar together until they look glossy and slightly thickened — about one minute. The mixture should feel warm from the butter and smell sweet and buttery.

Add the eggs and vanilla: Crack in the eggs one at a time, whisking vigorously after each addition. The batter will turn pale and creamy, and you’ll hear a soft slapping sound as it thickens. Stir in the vanilla extract.

Fold in the dry ingredients: Sift the flour, cocoa powder, and salt over the wet mixture. Use a spatula to fold gently — stop as soon as you can no longer see white streaks of flour. The batter should be thick, dark, and smell deeply of chocolate.

Pour and prepare the swirl: Scrape the brownie batter into the lined tray and spread it evenly with the spatula. In a small bowl, stir the Nutella with a tablespoon of milk if it feels stiff — it should be just thin enough to drizzle. Drop spoonfuls of Nutella across the surface in a random pattern.

Create the swirl: Take a butter knife or skewer and drag it through the Nutella and batter in a figure-eight motion. Don’t overdo it — six or seven passes are enough. You want distinct ribbons of Nutella, not a muddy mess. The surface should look marbled, with dark brown and lighter brown bands.

Bake: Slide the tray into the centre of the oven and bake for 22–25 minutes. At 20 minutes, check by inserting a toothpick into the centre — it should come out with a few moist crumbs clinging to it, not wet batter. The edges will look set and slightly cracked, and the kitchen will smell like a chocolate shop.

Cool completely: Place the tray on a wire cooling rack and let the brownies cool in the tray for at least 20 minutes. This is the hardest part — the brownies will continue to set as they cool. Lifting them too early will cause them to break apart. Once cool, lift the parchment overhang to transfer the slab to a cutting board.