Whisk the dry ingredients together: In a medium saucepan, sift the cocoa powder, cornflour, sugar, and salt. Whisk them together until the mixture looks uniform and no streaks of cocoa remain. You should see a fine, dark brown powder that smells deeply chocolatey.

Add a splash of milk to make a paste: Pour in about 60 ml of the milk and whisk vigorously until you have a smooth, thick paste. It should feel slightly gritty from the sugar but completely lump-free. This step prevents clumps when you add the rest of the liquid.

Pour in the remaining milk slowly: Add the rest of the milk in a steady stream while whisking constantly. The mixture will look thin and milky, like a chocolate drink. Keep whisking until it's fully combined and you see no dry bits stuck to the sides of the pan.

Cook over medium heat, stirring constantly: Place the saucepan over medium heat and switch to a wooden spoon or silicone spatula. Stir in a figure-eight motion, scraping the bottom and corners. After about 3 minutes, you'll feel the mixture start to thicken—it will feel heavier on the spoon and tiny bubbles will appear at the edges.

Watch for the first big bubble: When the pudding releases one or two large, slow bubbles that break the surface with a soft "pop," remove the pan from the heat immediately. The texture should be thick enough to coat the back of a spoon thickly, like a custard. If you draw a line through it with your finger, the line should hold without running.

Stir in the chocolate and vanilla: Add the chopped dark chocolate and vanilla extract while the pudding is still hot. Stir gently until the chocolate has melted completely. The mixture will become glossy and smell intensely rich. If using butter, stir it in now until it melts and the pudding looks silky.

Whisk in the protein powder off the heat: Let the pudding cool for 1 minute, then add the protein powder. Whisk vigorously to incorporate it fully. The mixture might seize slightly at first—keep whisking until it becomes smooth again. It will look slightly thicker than before and have a faint sweet aroma from the protein.

Portion and chill: Divide the pudding evenly into four ramekins or small bowls. Press a piece of cling film directly onto the surface of each pudding to prevent a skin from forming. Refrigerate for at least 2 hours, or overnight. The pudding will firm up to a spoonable, velvety consistency.