Liquefy the cornstarch in a little bit of port wine. Bring remaining port wine to boil with 1 tbsp cloves, sugar, mustard powder, lemon zest and coriander, add plums and steep for 10 min. Stir in starch mixture to thicken and bring to boil repeatedly.
In the meantime, boil milk with remaining sugar. Cut open vanilla pods, scrape out pith and add both pith and scraped pods to the saucepan. Boil well and steep for a few more min. Remove the vanilla pods. Mix the egg yolks, add a little hot milk and then add to the hot milk, stirring all the time. When the liquid starts to thicken, remove the saucepan from the stove and continue whisking heartily. Then strain through a sieve.
Pour the plum mixture into small gratin dishes, pour the vanilla foam over it and gratinate briefly.