For the doughnuts with coffee sauce, first stir the crumbled yeast with 1 tsp sugar into 60 ml milk. Let stand covered for about a quarter of an hour.
Mix the remaining sugar with flour, salt and cinnamon. Make a well in the center and pour the yeast into it. Add the remaining ingredients and knead into a smooth dough.
Grease a bowl with butter and let the dough take fat in it from all sides. Cover and leave to rise in a warm place for about an hour, knead briefly and then roll out to a thickness of about 1 1/2 cm. Cut out circles about 3 cm in size. Place on a baking sheet lined with parchment paper and let rise, covered, for another quarter of an hour.
In the meantime, grate the chocolate for the sauce. Whisk the egg yolks with 2 tablespoons of milk. Heat all the other ingredients, stirring constantly, but do not boil. Then stir in the chocolate and finally the egg yolks.Fry the doughnuts in enough hot oil (test with cooking spoon) until golden brown. Remove, drain on paper towels and roll in vanilla sugar.
Reheat the sauce and let it thicken for about 10 minutes. Serve the doughnuts with coffee sauce.