Preheat your oven to 170°C (fan-assisted). Line a 20cm square baking tin with parchment paper, leaving an overhang on two sides for easy lifting. The paper should feel snug against the edges — trust me, this makes all the difference later.

Break the dark chocolate into pieces and cut the butter into small cubes. Place both in a heatproof bowl set over a pan of barely simmering water. Stir gently with a spatula until everything is smooth and glossy — the mixture will look like satin as it comes together. Remove from the heat and let it cool for five minutes. You'll smell that deep, toasted cocoa scent rising up; it's the first sign that something wonderful is happening.

Whisk the caster sugar into the melted chocolate and butter mixture. The texture will become slightly grainy, but that's fine. Add the eggs one at a time, whisking vigorously after each addition until the batter is thick and falls from the whisk in a slow, velvety ribbon. Stir in the vanilla extract — the batter will now be pale and airy, almost mousse-like in consistency.

Sift the plain flour, cocoa powder, and salt together into a small bowl. Fold these dry ingredients into the wet mixture using a gentle hand — you want to see no streaks of flour, but stop the moment it comes together. The batter will be thick, dark, and smell intensely of chocolate. Overmixing will make the brownies tough, so treat this step like a quiet conversation, not a wrestling match.

Pour the brownie batter into the prepared tin and spread it into an even layer with a spatula. The surface should be smooth and level. In a separate small bowl, stir the Nutella with the vegetable oil until it becomes a little looser and more drizzle-friendly. Drop spoonfuls of this Nutella over the brownie batter in a random pattern — I like to do about eight to ten dollops.

Now for the magic: take a skewer or the tip of a knife and drag it through the Nutella and batter in long, sweeping curves. Move from one side of the tin to the other, then repeat in the opposite direction. You will see beautiful marbled patterns appear — the Nutella will swirl into the dark batter like ink through water. Don't over-swirl, or you'll lose the distinct ribbons. Two or three passes in each direction is plenty.

Bake for 28-32 minutes. At the 25-minute mark, check the brownies: the edges should look set and slightly pulling away from the tin, and the top will have a delicate, crackled crust. The centre should still have a slight wobble when you gently shake the tin — that's the promise of a fudgy interior. A skewer inserted into the centre should come out with a few moist crumbs attached, not clean. The smell at this point is utterly intoxicating — rich, toasty, and faintly nutty from the Nutella.

Place the tin on a wire cooling rack and let the brownies cool completely in the tin. This is the hardest part, but it's essential. As they cool, the structure sets and the flavours deepen. After about an hour, lift the brownies out using the parchment overhang and transfer them to a cutting board. Use a sharp knife to slice into 16 squares — wipe the blade clean between cuts for neat edges. The brownies will have a glossy, crackled top and a dense, fudgy interior that yields gently to the touch.