Place flour in a bowl and make a well in the middle.
In a small bowl, stir together warm milk with sugar and yeast until smooth. Pour into the flour well. Add salt, egg yolks and 25g butter. Work the mixture into a smooth dough. Cover with a tea towel and let rise in a warm, draught free area for 1 hour.
Meanwhile, mix together plum jam with rum and cinnamon. Set aside.
Divide the dough into 12 pieces. Press each piece down with the palm of your hand. Fill each piece with a dollop of plum jam and wrap the dough around the jam, forming a dumpling. Place on a floured board, cover with a tea towel and let rise for 25 minutes.
Bring two pots of salted water to the boil and place the yeast dumplings into the boiling water. Cover with a lid and bring to the boil again. Turn dumplings over and reduce heat. Continue to cook dumplings in barely simmering water for 12 minutes.
To check doneness, pierce with a fork. If the dough does not stick to the fork, the dumplings are ready. Remove dumplings with slotted spoon and drain well. Sprinkle with icing sugar and poppy seeds, then drizzle with melted butter.