Put the sugar in a large, deep saucepan with 1 litre of water and dissolve over a low heat, stirring constantly.
Leaving the stalk intact, peel the pears and remove the cores by cutting in through the base using a small knife. Sprinkle the lemon juice over the pears as soon as they are peeled, to prevent them from browning.
Split the vanilla bean in half, scrape the seeds into the sugar syrup, then add the bean. Bring the syrup to the boil, then add the pears. Reduce the heat and poach the pears gently for 15–20 minutes until they are just tender when tested with a skewer. Leave to cool in the syrup.
Shortly before serving, put the cream in a heatproof bowl. Add the chocolate and the liqueur, if you are using it, and heat over a saucepan of simmering water, stirring from time to time, until the chocolate has melted. (Alternatively, heat on low in a microwave oven.) Divide the hot sauce among serving bowls and sit a pear in each one. Serve immediately.