Melt the chocolate and butter together: Place the chopped dark chocolate and cubed butter in a heatproof bowl set over a pan of gently simmering water—make sure the bowl doesn't touch the water. Stir occasionally with a rubber spatula until the mixture is completely smooth, glossy, and silky. You'll know it's ready when it falls from the spatula in a thick, lazy ribbon and smells deeply of toasted cocoa.

Whisk in the sugars: Remove the bowl from the heat and add both the caster and light brown sugars. Whisk vigorously for about a minute until the sugar has mostly dissolved and the mixture looks slightly grainy but homogenous. The colour will darken to a rich, almost mahogany hue, and you'll hear the granules scraping against the bowl.

Add the eggs and vanilla: Crack the eggs into a small jug and beat them lightly with a fork. Pour them into the chocolate mixture along with the vanilla extract. Whisk for a full 60 seconds—the batter will thicken noticeably, become glossier, and develop a faint sheen on the surface. It should feel heavy and smooth as you stir.

Fold in the dry ingredients: Sift the plain flour, cocoa powder, and salt directly over the batter. Use a large metal spoon or a flexible spatula to fold everything together gently. Stop as soon as you can no longer see any streaks of flour—over-mixing will knock the air out and make the brownies tough. The final batter should be thick, fudgy, and smell intensely of rich chocolate.

Stir in the chocolate chips: Add the extra chocolate chips or chopped chocolate and fold them through until evenly distributed. You'll feel the occasional bump of solid chocolate against the spoon as you work.

Bake low and slow: Pour the batter into your lined tin and spread it evenly with a spatula—it will be quite thick, so take your time. Bake at 160°C (140°C fan) for 28-32 minutes. The brownies are done when the top looks set, cracked, and slightly dull, and a skewer inserted into the centre comes out with a few moist, fudgy crumbs attached—not wet batter. The edges will have pulled away slightly from the sides of the tin, and the kitchen will smell like a chocolate shop.

The crucial cooling stage: Place the tin on a wire rack and leave it to cool completely—this takes a good two hours at room temperature. Do not be tempted to rush this step. As the brownies cool, the crumb sets into that dense, fudgy texture you're after. Once cool, lift the brownies out using the parchment paper and slice into 16 squares with a sharp knife, wiping the blade clean between cuts for neat edges.