Make the dough: Drain the quark very well (press through a sieve or pat ricotta dry). In a bowl, mix quark, eggs, semolina, sugar, vanilla sugar (or vanilla), and a pinch of salt until smooth. Let rest 20 minutes so the semolina hydrates and the dough firms up.
Form and cook the dumplings: Bring a large pot of well-salted water to a lively simmer. Using two wet spoons, scoop and gently slide small portions of dough into the water. Cook until the dumplings float (about 10 minutes). They should feel softly set.
Toast the breadcrumbs: While the dumplings cook, melt butter in a skillet over medium heat. Add breadcrumbs and toast, stirring, until golden and aromatic. Stir in sugar and a little cinnamon to taste.
Finish and serve: Lift the dumplings with a slotted spoon, drain briefly, then roll in the buttery cinnamon-sugar breadcrumbs. Serve immediately.