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Whisk the double cream in a bowl until it holds its shape but is still pourable. Set aside.
Pour the milk or single cream into a saucepan with the dark chocolate. Stir continuously over a low-medium heat using a whisk until the mixture comes to the boil. Reduce the heat to low and simmer while continuing to whisk. Add the demerara sugar to taste.
Warm two heatproof glasses by rinsing with hot water, then dry and pour in the espresso. Carefully pour over the chocolate mixture, then top with the whipped cream, hazelnuts and a pinch of cocoa powder.