Melt the butter and chocolate: Place the butter and chopped dark chocolate in a heatproof bowl set over a pan of gently simmering water. Stir occasionally until the mixture is completely smooth and glossy. The smell at this stage should be deeply chocolatey, not burnt. Remove from the heat and let it cool for five minutes. It should feel warm to the touch, not hot.

Whisk in the sugar and eggs: Add the caster sugar to the chocolate mixture and whisk vigorously until fully incorporated. The mixture will look grainy. Add the eggs one at a time, whisking well after each addition. The batter will become thick, glossy, and should fall from the whisk in a smooth, thick ribbon.

Add the dry ingredients: Gently fold in the sifted plain flour, cocoa powder, and fine sea salt using a rubber spatula. Stop folding as soon as you see the last streak of flour disappear. The batter should be thick, fudgy, and smell intensely of chocolate.

Fold in optional white chocolate: If using, gently fold the white chocolate chips into the batter. Do not overmix; you want distinct pockets of white chocolate that will melt into creamy puddles.

Bake the brownies: Pour the batter into a prepared 20cm x 20cm square baking tin lined with parchment paper. Smooth the top with a spatula. Bake in a preheated oven at 170°C (fan-forced) for 22-25 minutes. The top should be set, cracked, and slightly shiny, but a skewer inserted into the centre should come out with a few moist, fudgy crumbs attached—not clean. The edges will look firmer than the centre.

Cool completely: Place the tin on a wire cooling rack and let the brownies cool completely in the tin. This is critical. Do not even think about cutting them for at least one hour. As they cool, they will set into that perfect, fudgy texture.

Chill for clean cuts: After cooling, transfer the entire block (still in the parchment) to the refrigerator and chill for at least 30 minutes. This firms the butter and makes for incredibly clean, sharp edges. I stumbled upon this combination by accident, and it's been a hit ever since.