Cover and simmer the plums, sugar, water and vanilla for approx. 6 mins. Remove the plums, reduce the juice for approx. 1 min. Return the plums to the pan, mix.
Chop the biscotti roughly, spread out in the dish. Spread the plum compote over the top. Combine the cream cheese, quark and sugar, fold in the whipped cream. Spread the cream over the plums, cover and chill for approx. 2 hrs. Dust the tiramisu with cocoa powder.