Finely chop 50 g milk chocolate on a chopping board.

Add 350 ml whole milk, 25 g caster sugar, 20 g muscovado sugar, and 150 ml double cream to the saucepan.

Heat the milk mixture over medium heat until steaming, without letting it boil.

Combine 35 g cornflour, 10 g cocoa powder, and the remaining 25 g caster sugar in one small mixing bowl.

Whisk 1 egg and 3 egg yolks together in the second small mixing bowl.

Whisk the cornflour mixture into the egg mixture until completely smooth.

Slowly pour the steaming milk mixture into the egg mixture while whisking constantly.

Pour the tempered custard mixture back into the saucepan.

Cook over medium heat, whisking continuously, until the custard thickens and begins to bubble.

Continue whisking the bubbling custard for 2 minutes, then remove the saucepan from the heat.

Stir 150 ml remaining double cream, 50 g chopped milk chocolate, and 1 g flaky sea salt into the hot custard.

Blend the custard with the hand blender until glossy and completely smooth.

Pour the custard into the heatproof container.

Press greaseproof paper directly onto the custard surface to prevent a skin from forming.

Chill the covered custard in the refrigerator overnight, at least 8 hours.

Beat the chilled custard lightly with a whisk before serving.