Bring the milk to the boil with 1 pinch of salt. Stir the rice into the boiling milk and let it swell at low heat for 30-35 minutes, stirring from time to time. Remove rice pudding from the heat and let it cool down a little.
In the meantime, preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease an ovenproof dish (approx. 1.75 l capacity). Beat the eggs, 75 g sugar, vanilla sugar and 1 pinch of salt with the whisks of the mixer until fluffy.
Stir in the quark, buttermilk and yoghurt. Mix rice pudding and quark mixture. Pour into the mould. Bake on the lower shelf in a hot oven for 35-40 minutes.
Wash, halve and stone the plums. Stir 5 tablespoons juice, 50 g sugar and sauce powder until smooth. Boil up the rest of the juice in a pot. Stir in the sauce powder, bring to the boil and simmer for about 1 minute.
Stir in the plums. Pour into a bowl and let cool.
Spread 2 tablespoons of butter in flakes on the casserole and bake for about 15 minutes. Mix 1 1⁄2 tbsp. sugar and 1 level tsp. cinnamon. Remove the casserole and sprinkle with cinnamon sugar. Add the plums.