Meanwhile, oil your baking tin, and line it with baking paper.

In a clean bowl, sift and mix the cocoa powder and cornstarch together. Then, add the sugar to this mixture. Add two ladles of the warm milk mixture to the dry ingredients, and stir until no more lumps remain. Then, pour this back into the remaining milk mixture, passing it through a sieve to remove any remaining lumps.

Add the roughly chopped dark chocolate into the milk mixture, then bring the whole thing to the boil, whisking it continuously. The mixture will become thick and gloopy as it cooks, and it should be one smooth, homogeneous paste.

Pour the mousse mixture into the lined baking tin, smooth the top with a spoon, and give it a few taps to make sure there are no air bubbles. Store this in the refrigerator for at least 2 hours, or overnight if possible.

When the mousse is completely cooled and set, flip it out onto a serving tray, then peel off and discard the baking paper. Dust on a thin layer of chocolate mousse on top, and you’re ready to slice it and serve!