Whisk the dry ingredients together: In a medium saucepan (off the heat), whisk the caster sugar, cocoa powder, cornflour, and sea salt until no lumps remain. The mixture should look like a fine, dark brown powder with a rich, slightly bitter chocolate smell. Breaking up any cocoa clumps now saves you a headache later.

Incorporate the milk slowly: Pour about 100 ml of the milk into the pan and whisk into a smooth paste. The texture will be thick and glossy, like a dark chocolate spread. Gradually pour in the remaining 400 ml of milk, whisking constantly. You should see a thin, even liquid with a uniform chocolate colour.

Add the egg yolk: Lightly beat the egg yolk in a small bowl, then stir it into the milk mixture. The liquid will turn a shade paler and feel slightly silkier to the whisk. This step gives the pudding its luxurious, custard-like finish.

Cook over medium heat: Place the pan over medium heat and begin stirring with a wooden spoon or silicone spatula. After about 2 minutes, you'll notice the mixture starting to steam gently. Continue stirring — don't walk away — as the pudding will thicken suddenly. You'll hear a soft bubbling sound as it reaches the right temperature.

Watch for the first bubbles: After 4–5 minutes, the mixture will release its first lazy bubbles around the edges. The colour deepens to a rich, dark brown, and the surface looks slightly shiny. Keep stirring; you're almost there. The spoon will start to leave a visible trail through the pudding as you stir.

Boil for one minute: Once the pudding is bubbling steadily all over, let it cook for exactly 1 minute while stirring. The mixture will thicken noticeably: it should feel heavy on the spoon and coat the back of a spoon thickly. When you drag your finger through the coating, the line should hold clearly without running. This is the moment the cornflour fully activates.

Remove from heat and add the chocolate: Take the pan off the heat immediately. Drop in the finely chopped dark chocolate and stir until it melts completely. The pudding will become even glossier and smell intensely chocolatey. If using butter, stir it in now for a mirror-like sheen.

Stir in the vanilla: Add the vanilla extract and give the pudding a final, vigorous stir. The aroma shifts from pure cocoa to a warmer, sweeter note. The pudding should be thick, smooth, and utterly inviting.

Divide and cool: Pour the pudding into four small serving bowls or ramekins. Tap each bowl gently on the counter to settle the surface. The texture should be thick but pourable — like a very rich hot chocolate that's decided to become a pudding. Let it cool at room temperature for 15 minutes, then refrigerate for at least 2 hours if you prefer a cold, set dessert. If serving warm, let it sit for just 5 minutes before digging in.