Prepare Your Tin: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Grease and line a 20cm x 30cm baking tray with parchment paper, leaving an overhang on the sides to easily lift the brownies out later.

Melt the Chocolate and Butter: In a large, heatproof bowl set over a pan of simmering water (bain-marie), combine the cubed butter and chopped dark chocolate. Stir occasionally until the mixture is smooth, glossy, and smells deeply chocolatey. Remove from the heat and let it cool slightly for a few minutes.

Combine Wet Ingredients: In a separate large bowl, whisk together the caster sugar and eggs until pale, thick, and slightly foamy – this might take about 2-3 minutes with a hand mixer, and the mixture will lighten in colour. Stir in the vanilla extract until just combined.

Fold in Chocolate Mixture: Pour the slightly cooled chocolate and butter mixture into the egg and sugar mixture. Gently fold with a spatula until just combined and streak-free; the batter will turn a rich, uniform dark brown and have a thick, luxurious sheen. Be careful not to overmix.

Add Dry Ingredients: Sift the plain flour, cocoa powder, and salt directly over the chocolate mixture. Gently fold until just combined. Stop as soon as you see no more dry streaks of flour; overmixing develops gluten and can lead to cakey, rather than fudgy, brownies. If using, fold in chocolate chips or chopped nuts now. The batter will be thick and shiny.

Bake to Perfection: Pour the batter into your prepared baking tray, spreading it evenly to the corners. Bake for 25-30 minutes. You're looking for a top that's set and slightly cracked around the edges, but the centre should still have a slight wobble when gently shaken. A skewer inserted into the centre should come out with moist crumbs attached, not wet batter. The smell filling your kitchen will be intensely chocolatey.

Cool Completely: Remove the tray from the oven and place it on a wire cooling rack. Allow the brownies to cool completely in the tin – this is crucial for that signature fudgy texture and clean cuts. The brownies will firm up considerably as they cool. Once cool, use the parchment paper overhang to lift them out, then slice into squares.