Put the water into a saucepan, then stir in the sugar, lemon juice and vanilla pod. Bring to a simmer and heat gently until the sugar has dissolved.
In the meantime, carefully peel the pears but leave the stalks intact. Then slice off a thin little disc at the base of each pear so that they can sit upright.
Put the pears into the hot syrup in the saucepan and cover with a piece of baking paper, to make sure the pears are submerged. Simmer the pears in the syrup for 20 minutes, then, using a small skewer, test them to see if they are tender. If not, cook for a further 5 minutes and test again.
When the pears are cooked, leave them to cool in the syrup.
To make the sauce, put the chocolate and cream into a heatproof bowl set over a pan of simmering water (make sure the base of the bowl doesn't touch the water). Stir constantly as the chocolate melts until the sauce is smooth and glossy.
To serve, drain the pears and serve them with the hot chocolate sauce, some vanilla ice cream or sweetened whipped cream and toasted sliced almonds on the top.