Preheat your oven to 160°C (320°F) fan-assisted. Line a 20cm (8-inch) square baking tin with parchment paper, leaving some overhang on two sides — this creates handles for lifting the brownie slab out later.

Melt the chocolate and butter together. Place the chopped dark chocolate and diced butter in a heatproof bowl set over a saucepan of barely simmering water. Stir gently with a silicone spatula until the mixture is completely smooth and glossy — you'll see a deep, rich sheen appear, and the smell of melted chocolate will fill your kitchen. Remove from heat immediately.

Stir in the condensed milk. Pour the sweetened condensed milk into the warm chocolate mixture. Stir in a slow, steady figure-of-eight motion until the batter becomes thick, velvety, and uniformly dark brown. It should feel heavy on the spoon, like a thick ganache.

Add the egg and vanilla. Crack the egg into a small bowl first (to avoid any shell mishaps), then add it along with the vanilla extract to the chocolate mixture. Whisk vigorously for about 30 seconds — you'll notice the batter lighten slightly in colour and become a bit looser. The texture should be silky and smooth, with no streaks of egg visible.

Sift in the dry ingredients. Add the sifted cocoa powder, plain flour, and salt. Fold gently with a spatula — use a cutting motion through the centre, then scrape around the bowl. Stop as soon as you see no more white flour streaks; overmixing will make the brownies cakey instead of fudgy. The batter will be thick, slightly sticky, and almost like a soft dough.

Fold in any extras. If using nuts or chocolate chips, scatter them over the batter and fold three or four times until evenly distributed. Don't overwork it — just until you see them scattered throughout.

Spread the batter into the prepared tin. Scrape the batter into the lined tin and use an offset spatula or the back of a spoon to spread it evenly into all corners. The surface should be level and smooth, with a slight gloss from the chocolate content.

Bake for 18-22 minutes. Place the tin on the middle rack. At 18 minutes, check by inserting a toothpick into the centre — you want it to come out with a few moist crumbs attached, not clean. The edges will look set and slightly cracked, while the centre will still jiggle gently when you shake the tin. Remove from the oven when you see the characteristic papery, cracked top forming.

Cool completely in the tin. Place the tin on a wire cooling rack and let the brownies cool for at least 30 minutes. This step is crucial — the brownies continue cooking from residual heat, and cutting them warm will result in a crumbly mess. You'll notice the top setting into a thin, crackly crust as they cool.

Lift out, slice, and serve. Use the parchment handles to lift the entire slab onto a cutting board. Use a sharp chef's knife to cut into 9 even squares — wipe the blade clean between cuts for neat edges. The brownies should be dense, rich, and almost fudge-like in texture, with a slight chewiness around the edges.