Put the flour in a bowl and make a well in the middle. Warm milk lukewarm. Add the yeast and dissolve it with 1 teaspoon of sugar. Pour the yeast milk into the cavity and cover and let rise in a warm place for about 10 minutes.
Add the rest of the sugar, egg yolk, butter, salt and lemon zest to the pre-dough and knead. Cover and let rise in a warm place until the dough has doubled in volume (approx. 30 minutes).
Knead the dough vigorously, cut in half and shape into 2 rolls. Then divide into 6 equal pieces, shape into balls and cover again for 30 minutes.
Heat oil or fat to 180 ° C. Fry the dough balls in portions, turning them. Take out, drain and let cool (preferably on kitchen towels).
Fill the jam into a piping bag with a small perforated nozzle, insert it into the side of the donuts and fill them with jam. Dust the donuts with powdered sugar or - if they are still slightly warm - turn them in normal sugar.
Note: Cherry jam or plum jam can be used as a filling. Eggnog also offers a delicious variety.