Heat the milk, cream and caster sugar together until just below boiling, stirring a little to help the sugar dissolve. Beat the egg yolks in a bowl with a wooden spoon. Slowly pour the cream and milk mixture on to the eggs, stirring the whole time. Pour this into a clean heavy-bottomed saucepan and heat over a low heat, stirring all the time. The mixture must not boil or the egg yolks will scramble, but you do need the mixture to thicken. It should be thick enough to coat the back of a spoon – if you run your finger along the spoon it should leave a clear channel.
Immediately take off the heat and add the chocolate, about 10 pieces at a time. Stir to help it melt. It’s really important that the chocolate melts completely. At first it won’t and you may panic a little, but keep stirring until you get there. Do not put it back on the heat. Put into a jug, cover and place the mixture in the fridge for a few hours to chill (overnight is even better). Only when the mixture is chilled can you stir in the vanilla and then the sherry.
Either churn in an ice-cream machine, following the manufacturer’s instructions, or still-freeze, putting it in a shallow container in the freezer and beating the mixture from time to time in a food processor or with an electric hand whisk. This ensures that the crystals are broken down and you end up with a smooth ice cream. If you’ve used a machine, transfer the ice cream to a bowl, cover with greaseproof paper and keep in the freezer.