Preheat and prepare. Set your oven to 180°C (160°C fan) and line a 20cm square baking tin with parchment paper, leaving some overhang on two sides. This makes lifting the whole slab out a breeze later.

Melt the butter. In a large microwave-safe bowl, melt the 150g of butter in 20-second bursts until it's fully liquid. It should look glossy and golden, and you'll smell that nutty, warm aroma as it melts. Set it aside for a minute to cool slightly — you don't want it hot enough to scramble the eggs.

Stir in the sugars. Add the caster sugar and light brown sugar to the melted butter. Stir vigorously with a wooden spoon until the mixture looks like wet sand and the sugar has mostly dissolved. The mixture will feel slightly gritty against the spoon, but that's fine.

Add the eggs and vanilla. Crack in the two eggs and pour in the vanilla extract. Whisk by hand for about one minute until everything is smooth, glossy, and a shade paler than when you started. The batter should look thick and ribbon-like when you lift the whisk — that's your cue that it's aerated enough.

Sift and fold the dry ingredients. Sift the plain flour, cocoa powder, and salt directly over the wet mixture. Fold gently with a silicone spatula — you want to see no streaks of white flour, but stop as soon as it comes together. Over-mixing will make the brownies tough rather than fudgy. The batter will look thick, dark, and almost velvety.

Fold in the extras. If using chocolate chips or nuts, scatter them over the batter and fold them through with three or four turns of the spatula. The chips should be evenly distributed, and you'll see little flecks of chocolate dotted through the dark batter.

Bake to perfection. Pour the batter into the prepared tin and spread it into an even layer with the spatula. Bake for 22–25 minutes. The top should look set and slightly cracked, but the centre will still wobble just a little when you gently shake the tin. A skewer inserted into the middle should come out with a few moist crumbs attached — never clean, as that means they're over-baked.

Cool completely before cutting. Place the tin on a wire cooling rack and leave the brownies to cool for at least 20 minutes. Use the parchment overhang to lift the whole block out onto a chopping board. Let them cool fully — about another 30 minutes — before slicing into squares. Cutting while warm gives you messy edges, though I don't mind those as a chef's treat.