For the mousse, possibly heat milk the day before. Break the chocolate into pieces and melt in a hot water bath. Leave in a hot water bath. Allow the milk to soak in very slowly, stirring constantly.
Cool down for about 30 minutes.
Whip the cream until stiff and carefully fold into the chocolate. Cover the mousse and chill for at least 3 hours.
For the plums, possibly wash and stone the fruit the day before. Bring 200 ml water, port wine, vanilla sugar, sugar and cinnamon to the boil. Stir the starch and 3-4 tbsp. water until smooth and use to thicken the wine stock.
Add plums, bring to the boil and simmer for 2-3 minutes. Leave to cool.
Shape the mousse into balls as desired with a large ice-cream scoop. Serve with some plum compote and add the rest.