Put the flour in a bowl. Press a depression in the middle and crumble the yeast into it. Sprinkle 80 g sugar around the yeast. Heat milk lukewarm and pour half of it to the yeast and mix to a pre-dough.
Cover and leave to rise in a warm place for about 15 minutes. In the meantime, melt the fat in the remaining milk at low heat. Add egg and salt and mix everything together. Add the fat-egg-mixture to the pre-dough and mix everything with the dough hooks of the hand mixer to a smooth yeast dough.
Cover and leave to rise in a warm place until the dough has doubled. Knead again and divide into 12 portions. Form a ball from each portion of dough and place on a baking tray. Cover and leave to rise for another 15 minutes.
Mix the remaining sugar and cinnamon. Heat vegetable fat in a wide pot. Fry 3-4 dough balls at once. To do this, press the dough balls slightly flat and use a skimmer to slide them into the hot fat.
Fry the underside until golden brown, then turn the doughnuts with the handle of a wooden spoon and finish baking. Remove the doughnuts from the fat with a skimmer, drain well and roll them immediately in the sugar-cinnamon mixture.
Put the plum puree in a piping bag with a very small perforated spout. Prick the nozzle sideways into the doughnuts and squirt the plum puree into it. Let the doughnuts with plum jam filling cool completely before serving.
Makes 12 pieces.