Preheat the slow cooker if necessary; see the manufacturer's instructions. Pour the milk and cream into a saucepan and bring just to the boil. Remove from the heat, add the chocolate pieces and set aside for 5 minutes, stirring occasionally, until the chocolate has melted.
Put the whole eggs, egg yolks, sugar and cinnamon in a mixing bowl and whisk until smooth. Gradually whisk in the warm chocolate milk, then strain the mixture into 4 heatproof pots or mugs, each 250 ml (8 fl oz).
Cover the tops of the pots or mugs with foil and stand them in the slow cooker pot. Pour hot water into the slow cooker pot to come halfway up the sides of the pots or mugs. Cover with the lid and cook on low for 3–3½ hours or until set.
Lift the dishes carefully out of the slow cooker pot using oven gloves. Leave to cool at room temperature, then transfer to the refrigerator for at least 4 hours until well chilled.
Just before serving, whip the cream for the topping until soft swirls form. Gradually whisk in the liqueur, then spoon the flavoured cream over the top of the desserts. Sprinkle with chocolate curls and serve.