Preheat your oven to 160°C (140°C fan) and line your baking tin. Grease a 20cm square baking tin with a little butter, then line it with a long strip of parchment paper, leaving an overhang on two opposite sides—this will act as handles to lift the brownies out later. The smell of warm butter on the paper is the first hint of what's to come.

Melt the butter and chocolate together gently. Place the chopped butter and dark chocolate in a heatproof bowl set over a saucepan of simmering water (make sure the bowl doesn't touch the water). Stir occasionally with a rubber spatula until the mixture is smooth and glossy. You'll see the butter melt first, followed by the chocolate turning into a thick, dark river. The aroma at this stage is deeply, almost intoxicatingly chocolatey. Remove from the heat and let it cool for five minutes—it should feel warm, not hot, to the touch.

Whisk in the sugar, eggs, and vanilla. Pour the sugar into the melted chocolate mixture and stir until combined. The mixture will look slightly grainy at first, but that's fine. In a separate small bowl, lightly beat the eggs and vanilla extract together, then pour them into the chocolate-sugar mixture. Whisk vigorously for a full minute—the batter will become thicker, shinier, and noticeably lighter in colour as the eggs incorporate. You should hear a soft, slick sound as you whisk, and the batter will fall in a thick ribbon from the whisk.

Fold in the dry ingredients. Sift the plain flour, cocoa powder, and salt directly over the batter. Using a flexible spatula, fold the dry ingredients into the wet ones with a gentle, sweeping motion. Stop as soon as you no longer see streaks of flour—over-mixing will develop the gluten and make the brownies cakey rather than fudgy. The batter at this point should be thick, dark, and smell intensely of rich cocoa.

Pour the batter into the prepared tin and bake. Scrape the batter into the lined tin and spread it evenly into the corners with the spatula. If you're adding chocolate chips or nuts, scatter them over the top now and press them in lightly. Bake for 30-35 minutes. The cue for doneness is crucial: the top should look set and slightly cracked, and a skewer inserted into the centre should come out with a few moist crumbs clinging to it—not wet batter. The edges will have pulled away from the sides of the tin slightly, and your kitchen will smell like a patisserie.

Cool completely in the tin, then chill for the best texture. Place the tin on a wire cooling rack and let the brownies cool to room temperature for at least one hour. For the ultimate fudgy texture, once cool, cover the tin with cling film and refrigerate for at least two hours, or overnight. This step is not optional—it allows the brownies to set properly and develop that dense, almost truffle-like centre. The brownies will feel firm to the touch and slice cleanly.

Slice, warm slightly, and serve. Lift the brownies out of the tin using the parchment paper overhang. Place them on a cutting board and use a sharp, dry knife to cut into 12 squares (wipe the blade clean between cuts for neat edges). For serving, warm each brownie in the microwave for 10-15 seconds, or in a low oven for 5 minutes, until just soft and warm. Place one brownie in a shallow bowl, top with a generous scoop of vanilla ice cream, and finish with a tiny pinch of flaky sea salt if you're using it. Watch the ice cream begin to melt over the warm brownie—that's the moment it's ready.