Break up the chocolate and melt it in a bain-marie with the chopped butter; stir with a whisk to mix thoroughly. Pour this melted mixture into a bowl, add the chestnut jam and stir. Now add the sugar, Vin Santo and a tablespoon of grated orange peel, stirring constantly. Line a loaf tin with cling film, pour in the mixture and level it well with the back of a spoon dipped in water. Fold the cling film in over the top to seal the tin and put it in the fridge for at least 12 hours. When it’s time to serve it, lift the dessert out using the cling film, turn it upside down on a serving dish and then remove the film. Dust with cocoa powder and serve.