Heat oven to 200C/180C fan/gas 6. Peel, halve and core the pears, then put them in a gratin dish or roasting tin in which they can sit without too much surface area around them (otherwise the juices that come out of them will just burn off ). Sprinkle with the sugar and drizzle over 4 tbsp sherry and the melted butter. Bake for 30-40 mins; the pears should be completely tender and the juices around them slightly caramelised (check on them after 20 mins – if the pears are small it will take less time). Cut the pears up – still in the dish – and pour over the remaining sherry. Leave to cool completely, then purée (including all the juice) and push through a sieve. Cover and chill.

To make the ice cream base, put the cream and milk in a heavy-based pan and add half the sugar. Slit the vanilla pod along its length and scrape the seeds out into the pan. Drop in the pod as well. Bring to just under the boil, stirring, then take the pan off the heat and leave to infuse. Remove the vanilla pod.

Put the egg yolks in a bowl with the rest of the sugar and whisk with electric beaters until the mixture has increased in volume and falls in ribbons (this will take about 2 mins). Gradually stir about half the cream mixture into the eggs and sugar, then add this to the rest of the cream in the pan. Set the pan over a very low heat and cook, stirring all the time with a wooden spoon, for 8-10 mins, or until the mixture has thickened enough to coat the back of a spoon. Be really careful not to let the mixture overheat or it'll curdle.

Pour the custard into a bowl set in a sink of iced water (this stops it cooking immediately) and leave to cool, stirring a little every so often. Chill for 3 hrs. Stir the pear purée and the 2 tbsp sherry into the custard, then churn in an ice cream maker following the manufacturer’s instructions. Spoon into a container, cover and store in the freezer. Alternatively, if you don’t have an ice cream maker, put the custard and pear mixture into a freezer-proof bowl, cover and put it in the freezer. Beat the mixture every hour until it’s frozen (about 4 hrs) to break down the crystals.