Preheat the oven to 180°C (fan) or 200°C (conventional). Grease and line a 33x23cm baking tray with baking parchment. The paper should hang over the two long sides — this creates handles for lifting the cake out later.

Melt the butter, cocoa, and water together. Place the 250g butter, 100g cocoa powder, and 400ml water in a large saucepan over medium heat. Stir occasionally with a wooden spoon until the butter has completely melted and the mixture is smooth and dark, like liquid chocolate. You will smell a deep, roasted cocoa aroma when it is ready — this takes about 3-4 minutes. Remove from the heat.

Whisk in the dry ingredients. Add the 400g plain flour, 400g caster sugar, 1 teaspoon bicarbonate of soda, and 1 teaspoon salt directly into the warm butter-cocoa mixture. Stir vigorously until no white streaks remain. The batter will be quite thick and smooth, with a glossy sheen. Let it cool for 2 minutes — you should be able to hold your hand an inch above the surface without feeling intense heat.

Add the wet ingredients. In a small bowl, lightly beat the 2 eggs with a fork. Pour them into the batter along with the 120ml buttermilk and 2 teaspoons vanilla extract. Stir until everything is fully incorporated. The batter will loosen significantly and become pourable, with a silky, fluid consistency. It should smell richly chocolatey with a faint tang from the buttermilk.

Bake the cake. Pour the batter into the prepared tin and spread it evenly with a spatula. Bake for 22-25 minutes. The cake is done when the top looks set and matte, and a skewer inserted into the centre comes out with a few moist crumbs attached — not wet batter. The edges will pull away slightly from the sides of the tin.

Make the icing while the cake bakes. About 5 minutes before the cake is finished, combine the 120g butter, 60g cocoa powder, and 80ml whole milk in the same saucepan you used earlier. Place over medium heat and stir until the butter melts and the mixture is smooth and bubbly around the edges. Remove from the heat and whisk in the 500g icing sugar and 2 teaspoons vanilla extract until the icing is thick, glossy, and lump-free. If using nuts, stir them in now.

Ice the warm cake. As soon as the cake comes out of the oven, pour the warm icing over the top. Use a spatula to spread it evenly — the heat from the cake will help the icing flow to the edges. You will see the icing begin to set into a fudge-like layer as it cools. Let the cake cool completely in the tin before slicing, about 1 hour, or serve slightly warm for a gooey texture.