Prepare your tray and preheat the oven. Preheat your oven to 180°C (160°C fan). Line a 20cm square baking tray with parchment paper, leaving an overhang on two opposite sides—this will act as handles to lift the brownie out later. You'll hear the paper crinkle as you press it into the corners; make sure it sits flat against the bottom and sides.

Melt the butter and chocolate together. Place the chopped butter and dark chocolate in a heatproof bowl set over a saucepan of simmering water. Stir gently with a rubber spatula until both are fully melted and the mixture is glossy and smooth. The colour will turn a deep, dark brown, and you'll smell the rich cocoa notes rising up. Remove from the heat and let it cool for five minutes—it should feel warm to the touch, not hot.

Whisk in the sugar, eggs, and vanilla. Add the caster sugar to the melted chocolate mixture and whisk until combined. The mixture will look grainy at first, then become thicker and more cohesive. Add the eggs one at a time, whisking vigorously after each addition until the batter is thick, shiny, and falls from the whisk in a slow, silky ribbon. Stir in the vanilla extract.

Fold in the dry ingredients. Sift the plain flour, cocoa powder, and fine sea salt directly over the batter. Using a spatula, fold gently in a figure-eight motion until just combined. Stop as soon as you no longer see streaks of flour—the batter should be thick, fudgy, and smell intensely of chocolate. Over-mixing will make the brownies tough and cakey, so be gentle.

Pour the batter into the tray and dollop the Nutella. Scrape the brownie batter into the prepared tray and spread it evenly with the spatula. The surface should be smooth and level. Spoon the Nutella over the top in five or six generous dollops, spacing them out evenly. You should see distinct mounds of hazelnut spread sitting on top of the dark batter.

Create the swirl pattern. Take a skewer or the tip of a butter knife and drag it through the Nutella dollops, connecting them in a continuous figure-eight or zigzag motion. Don't over-swirl—two or three passes in each direction is enough. You want clear, defined ribbons of Nutella against the dark brownie base. If you swirl too much, the Nutella will blend into the batter and you'll lose the visual impact.

Bake to perfection. Place the tray in the centre of the oven and bake for 25 minutes. At the 20-minute mark, peek through the oven door—the top should be set, slightly crackled, and the edges will be pulling away from the parchment paper. A skewer inserted into the centre should come out with a few moist crumbs attached, not wet batter. The smell of toasted hazelnut and chocolate will fill your kitchen. Let the brownie cool completely in the tray on a wire rack—this is crucial for clean, restaurant-quality slices.

Lift, slice, and serve with flair. Once completely cool, use the parchment overhang to lift the entire brownie slab onto a cutting board. Use a sharp, large knife to trim the edges if you want perfect squares, then cut into 16 pieces. For the most dramatic presentation, warm each slice briefly in the microwave (10 seconds), place it on a plate, add a scoop of vanilla ice cream, and finish with a sprinkle of flaky sea salt.