Preheat and prepare your tin. Preheat your oven to 180°C (160°C fan). Line a 20cm square baking tin with parchment paper, leaving some overhang on two sides — this creates handles for lifting the whole slab out later. The paper should sit flush against the sides, no air pockets.

Melt the chocolate and butter together. Place the chopped dark chocolate and cubed butter in a large, heatproof bowl. Microwave in 30-second bursts, stirring well after each, until completely smooth and glossy. The mixture should look like thick, dark silk and smell intensely of chocolate. Set aside for 2 minutes to cool slightly — you don’t want to scramble the eggs next.

Whisk in the sugars and eggs. Add the caster sugar and light brown sugar to the melted chocolate mixture. Whisk vigorously until the sugar begins to dissolve and the mixture looks grainy but uniform. Crack in the eggs one at a time, whisking thoroughly after each addition. Add the vanilla extract. The batter will become thick, glossy, and start to pull away from the sides of the bowl. You’ll notice it looks almost like a thick pudding — that’s exactly what you want.

Fold in the dry ingredients. Sift the plain flour, cocoa powder, and salt directly over the batter. Use a spatula to fold gently — cut through the centre, scrape the bottom, and turn the mixture over. Stop as soon as you no longer see streaks of flour. Overmixing will develop gluten and give you cakey, not fudgy, brownies. The final batter should be thick, slightly stiff, and have a gorgeous, dark velvety sheen.

Add the chocolate chips (if using). Fold in the chocolate chips or chunks with just three or four turns of the spatula. They should be evenly distributed without overworking the batter.

Bake to perfection. Pour the batter into the prepared tin and spread it evenly with the spatula. Bake for 22–25 minutes. The top should look set, dry, and slightly crackly, with a few tiny bubbles around the edges. The real test: insert a skewer into the centre — it should come out with a few moist crumbs clinging to it, not wet batter. If it’s clean, you’ve overbaked them. The brownies will continue to set as they cool, so err on the side of underdone.

Cool completely before slicing. Place the tin on a wire cooling rack and let the brownies cool for at least 20 minutes. Using the parchment overhang, lift the entire slab onto a chopping board. Let it cool for another 10–15 minutes before slicing into 16 squares with a sharp knife. For clean cuts, wipe the blade between each slice. The edges will be slightly firmer, the centres soft and fudgy — the perfect contrast.