Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the chocolate and butter in a bowl, set over a saucepan of very gently simmering water making sure that the water does not touch the base of the bowl and leave until just melted.
Meanwhile, butter 4 mugs, each 250 ml (8 fl oz), checking first that they will fit in the slow cooker pot.
Whisk the egg whites in a bowl until soft peaks, then gradually whisk in the sugar a teaspoonful at a time until it has all been added and the meringue is thick and glossy. Take the bowl of chocolate off the saucepan, stir in the egg yolks and warm water and gently fold in a spoonful of the egg whites to loosen the mixture. Fold in the remaining egg whites, then divide between the mugs.
Cover the tops with domed foil and put into the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the mugs. Cover with the lid and cook on high for 1–1¼ hours or until the puddings are softly set in the centre.
Lift the puddings out of the slow cooker pot and remove the foil. Mix the cream and mint together and pour into a jug. Dust the puddings with icing sugar and serve immediately with the mint cream.