Heat milk for the yeast dough (approx. 38 ° C). Add the fresh yeast and a teaspoon of the sugar and stir. Leave covered for about 5 minutes. Mix the flour, remaining sugar, vanilla sugar and salt in a large bowl. Form a well and add the milk-yeast mixture and the egg.
Knead the ingredients into a dough with your hands for about 8 minutes. This works best on a clean work surface. Roll the dough into a ball and place in the bowl dusted with flour. Sprinkle some flour on the dough, cover with a tea towel and let it rise in a warm place, preferably without a draft, for 45 minutes.
In the meantime, for the cherry filling, bring the juice from the sour cherry glass to boil with cornstarch and sugar, stirring constantly, until it thickens. Add the sour cherries and stir in with a spoon. Transfer the cherry mixture and let it cool down covered with cling film.
Now prepare the vanilla sauce. To do this, scrape out the pulp of the vanilla bean with the back of your knife. Bring them to a boil together with the milk in a saucepan, stirring constantly. Mix the egg yolks with the sugar and add to the milk. Allow to simmer gently while stirring until the desired creaminess is achieved. Cover the pudding directly on the surface with cling film and let it cool.
Remove the leavened yeast dough from the bowl, divide into seven equal pieces (approx. 70 grams each) and roll into balls. Spread each ball with your hands on the work surface. Place 1 – 1.5 teaspoons of cherry filling on each of the 1.5 cm thick circles in the middle and close them. Place the closed side down on the floured work surface, cover with a tea towel and let it rise again for 30 minutes.
Bring water, salt and oil to a simmer in a high, coated pan. Place the risen yeast dumplings in the pan with the help of a spatula. Close the lid and steam at medium high for 18 to 20 minutes. The salt crust forms in the last minute.
Serve the yeast dumplings with vanilla sauce. Mix the blue poppy seeds with icing sugar and sprinkle on the vanilla sauce.