Sift flour into a bowl. Sprinkle sugar around the edges of the flour. Make a well in the middle and crumble the yeast into it. Mix with a little lukewarm milk, flour and sugar. Cover the bowl with a cloth, put it in a warm place and let the dough rise. After 15 minutes, the yeast dough will have doubled in size.
Mix the egg with the remaining milk. Add to the flour mixture. Add soft butter in flakes, lemon zest and salt. Knead everything well. Punch the dough down on a floured baking board until it bubbles. Cover and let rise in a warm place.
Knead again and cut balls of about 4 cm with a glass. Let the balls rise on a warm, floured board for another 30 minutes, covered, in a draft-free, warm place.
Pour about 2 cm of milk into a large saucepan or roaster, add butter, sugar and salt and heat.
Place the risen yeast dumplings side by side in the warm milk and close the saucepan tightly so that no steam can escape. Let it boil briefly, then steam at low temperature for 30-45 minutes. When it starts to crackle in the cooking pot, it is the sign that the Buchteln have absorbed the milk and are crispy brown at the bottom.
Place the yeast dumplings brown side up on a hot plate and serve with sweet compote, roaster or vanilla sauce.