In a medium bowl, whisk together the caster sugar, cocoa powder, cornflour, and sea salt until no lumps remain. The mixture should look like fine, dark sand with a uniform colour.

In a separate small bowl, lightly beat the egg yolks with a fork until they are pale yellow and slightly frothy—this takes about 30 seconds.

Pour the milk into a heavy-based saucepan and place over a medium heat. Watch for tiny bubbles forming around the edge of the pan; you want it steaming hot but not boiling, which takes roughly 3-4 minutes.

Slowly pour about 100ml of the hot milk into the dry ingredients while whisking constantly. The mixture will turn into a thick, dark paste that smells intensely of chocolate.

Pour the paste back into the saucepan with the remaining milk, whisking continuously. Return to a medium heat and stir with a wooden spoon. After 2-3 minutes, you will feel the mixture starting to thicken; it will make a soft, slapping sound against the sides of the pan.

Once the pudding has thickened to the consistency of double cream—about 5 minutes of gentle simmering—remove from the heat. Quickly whisk in the beaten egg yolks, vanilla extract, and finely chopped dark chocolate. The heat will melt the chocolate, and the pudding will turn glossy and dark, like liquid velvet.

Stir in the unsalted butter until completely melted and incorporated. The surface should look smooth and shiny, with no graininess visible.

Pour the pudding into individual serving dishes or one large bowl. Press a piece of cling film directly onto the surface to prevent a skin forming. Refrigerate for at least 2 hours, or until completely chilled and set. When ready, the pudding should wobble gently but hold its shape when spooned.