Preheat the slow cooker if necessary; see the manufacturer's instructions. Whisk the egg yolks and sugar in a bowl for 3–4 minutes until frothy, then gradually whisk in the cream. Stir in the peppermint extract, then strain the egg custard into a jug.
Pour into 4 ramekin dishes, each 150 ml (¼ pint), checking first that they will fit in the slow cooker pot. Put the dishes into the slow cooker pot, pour boiling water into the pot to come halfway up the sides of the dishes, then loosely cover the top of each dish with foil.
Cover with the lid and cook on low for 2½–3½ hours or until the custard is set with a slight quiver to the middle. Lift the dishes carefully out of the slow cooker and leave to cool. Transfer to the refrigerator to chill for 4 hours.
When ready to serve, pile a few raspberries in the centre of each dish and sprinkle over some icing sugar. Caramelize the sugar with a cook's blow torch.