Prepare Your Tin: Preheat your oven to 180°C (160°C fan/Gas 4). Grease a 20cm square baking tin and line it with a single piece of baking parchment, leaving an overhang on two opposite sides. This will act as a 'sling' to lift the brownies out easily later.

Melt the Chocolate and Butter: Place the cubed butter and broken dark chocolate in a medium, heatproof bowl. Set the bowl over a saucepan of barely simmering water (a bain-marie), making 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 about 10 minutes.

Sift Dry Ingredients: While the chocolate cools, sift the plain flour, cocoa powder, and sea salt into a separate bowl. Sifting is important as it removes lumps and aerates the mixture, leading to a more uniform texture. Give it a quick whisk to combine.

Whip the Eggs and Sugar: In a large bowl, using an electric hand mixer or a stand mixer with the whisk attachment, beat the eggs and caster sugar together on high speed. Continue whisking for 5-8 minutes, until the mixture is very pale, thick, and has at least doubled in volume. When you lift the whisk, it should leave a ribbon-like trail on the surface for a few seconds. This step is vital for the crackly top.

Combine the Mixtures: Gently pour the slightly cooled chocolate-butter mixture into the whipped eggs and sugar. Add the vanilla extract. Using a large spatula, carefully fold the mixtures together until just combined. You want to keep as much air in the eggs as possible, so be gentle.

Fold in the Dry Ingredients: Add the sifted dry ingredients to the wet mixture in two batches. Fold them in gently with the spatula until you no longer see any streaks of flour. What works best for me is to stop folding just before it's fully combined to avoid over-mixing.

Add Chocolate Chips (Optional): If you're using them, gently fold in the chocolate chips now. Be careful not to over-mix the batter at this stage.

Bake to Perfection: Pour the glossy batter into your prepared tin and smooth the top with your spatula. Bake for 20-25 minutes. The edges should be set and pulling away from the sides slightly, and the top should be crackly. A skewer inserted into the centre should come out with moist, sticky crumbs attached, not wet batter.

Cool Completely: This is the hardest part! Leave the brownies to cool completely in the tin on a wire rack. This allows them to set properly, developing their dense, fudgy texture. Don't even think about cutting them while they're warm. Once cool, use the parchment paper sling to lift the brownie slab out of the tin. Cut into 16 squares with a sharp knife.