Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the cocoa and instant coffee in a bowl and mix to a smooth paste with a little of the boiling water.
Pour the cocoa paste into the slow cooker pot. Add the remaining boiling water, the rum, sugar, cinnamon and red chilli and mix together. Cover with the lid and cook on low for 3–4 hours until piping hot or until the coffee is required.
Stir well, then ladle into heatproof glasses. Whip the cream until it is just beginning to hold its shape and spoon a little into each glass. Decorate each drink with a little grated chocolate and a dried chilli, if liked.