Warm the milk in a pot. Roughly chop the chocolate and melt in a hot water bath. Leave the chocolate on the water bath and let the milk slowly soak in while stirring constantly. Let it cool down.
Whip the cream until stiff. Carefully fold into the chocolate. Cover the mousse and chill for about 3 hours.
For the compote, wash, halve and stone the plums. Bring 200 ml water, port wine, sugar, vanilla sugar and cinnamon stick to the boil in a pot. Stir starch with 3 tbsp. water until smooth. Stir into the port wine mixture and bring to the boil again while stirring.
Add plums, bring to the boil and simmer for 2-3 minutes. Pour into a bowl and let it cool down.
Shape the mousse into balls with a large ice-cream scoop or form cams with two tablespoons. Arrange some plum compote with the mousse. Add the rest of the compote.