For the mousse: Stir the rum and gelatine together in a cup, and let soften 5 minutes. Heat a third of the cream with the vanilla in a saucepan. Stir in the gelatine and rum to dissolve. Add the sugar to dissolve. Finally mix in the pumpkin puree. Strain, cool, and refrigerate until chilled and starting to get thick. Whip the remaining cream and fold it into the chilled pumpkin mixture. Spoon into serving dishes, cover, and chill at least 4 hours before turning out to serve. For the chocolate sauce: put the ingredients in a saucepan and gently melt. If too thick, simply thin out with a little more cream. The sauce will harden as it cools, but you can simply melt it down again. Serve the chocolate sauce dribbled over the pumpkin mousses.
To make the pumpkin puree: Preheat the oven to 400 degrees F. Toss the diced pumpkin with the butter in a baking dish. Cover the dish lightly with foil, and roast until very soft, about 30 minutes, depending on the size of the chunks. Puree the pumpkin in a blender.