Prepare Your Tin and Oven: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Grease a 20cm (8-inch) square baking tin and line the base and sides with a single piece of baking parchment, leaving some overhang to help you lift the brownies out later.

Melt the Chocolate and Butter: Place the cubed butter and broken dark chocolate in a heatproof bowl set over a saucepan of barely simmering water (a bain-marie). Make sure the bottom of the bowl doesn't touch the water. Stir occasionally until everything is completely melted and smooth. Remove from the heat and set aside to cool slightly for 10-15 minutes.

Whisk the Eggs and Sugar: In a separate large bowl, using an electric hand mixer or a stand mixer with the whisk attachment, beat the eggs, caster sugar, and vanilla extract together on high speed. I find that whisking for a good 5-7 minutes is essential. The mixture should become very pale, thick, and almost double in volume. You'll know it's ready when it leaves a ribbon-like trail on the surface when you lift the whisk.

Combine Wet Ingredients: Gently pour the slightly cooled chocolate and butter mixture into the whisked egg mixture. Using a spatula, carefully fold them together until just combined. Be gentle here to avoid knocking out all the air you just incorporated.

Fold in the Dry Ingredients: Sift the plain flour, cocoa powder, and salt directly over the bowl. Fold everything together gently until you can no longer see any streaks of flour.

Add Extras: If you're using them, stir through the chocolate chips or chopped nuts now.

Bake the Brownies: Pour the batter into your prepared tin and smooth the top with your spatula. Bake on the middle shelf of the preheated oven for 25-30 minutes.

Check for Doneness: To check if they're ready, insert a skewer or cocktail stick into the centre. It should come out with moist, sticky crumbs attached. If it comes out with wet batter, they need a few more minutes. If it comes out clean, they may be overbaked.

Cool Completely: This is a crucial step! Leave the brownies to cool completely in the tin, first on a wire rack for 30 minutes, and then you can pop them in the fridge for an hour to firm up if you like. This ensures a clean cut and a perfectly fudgy texture. Once cool, use the parchment paper overhang to lift the slab out and cut it into 16 squares.