Drain plums well in a sieve, collecting the juice. Stir 5-6 tbsp. plum juice and starch until smooth. Boil up the rest of the plum juice, orange juice, cinnamon and 1 tbsp. sugar. Stir in the starch, bring to the boil again and simmer for about 1 minute while stirring.
Remove from the heat and carefully stir in the plums. Let them cool down.
Mix coffee and port wine. Line a large rectangular form with the cantuccini. Sprinkle with the coffee mixture.
Separate eggs. Beat the egg whites until stiff and cover and chill. Whisk the egg yolks, vanilla sugar and 50 g sugar with the whisks of the mixer for 4-5 minutes until thick and creamy. Stir in the quark and mascarpone.
Finally fold in the beaten egg white with a whisk.
Spread half of the cream evenly on the cantuccini. Spread plum compote on top. Spread the rest of the cream on top and chill for at least 1 1/2 hours. Dust with cocoa shortly before serving.