First of all, pour 2½ pints (1.5 litres) water into the saucepan, then stir in the sugar, lemon zest and juice, along with the vanilla pods.
You can also watch our Cookery School Video on zesting citrus fruits on this page. Bring everything up to simmering point and heat gently until the sugar has dissolved. While this is happening, using a potato peeler, thinly pare off the outer skin of 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.
Next, carefully remove a little of the core from the base of each pear, using the pointed end of the peeler. Now put the pears into the hot syrup in the saucepan and cover with a piece of baking parchment, which needs to cover the liquid 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 them again.
When the pears are cooked, leave them to cool in the syrup (they can be stored in the syrup in an airtight, polythene container in the fridge for a couple of days). To make the sauce, put the chocolate, cream and liqueur into a heatproof bowl set over a pan of barely simmering water (making 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, and some vanilla ice cream or whipped cream. On very special occasions, Alain serves the pears with crème Chantilly – whipped cream that has been sweetened with vanilla sugar – and scatters toasted flaked almonds over the top.