Mix 300 g of the 400 g flour with the yeast, sugar, salt and cinnamon in a large bowl. Heat the milk with the butter in a saucepan (not over 50 degrees!). Add to the flour mixture, add the egg yolks and knead for 2 minutes with a hand mixer on low speed, adding the remaining flour. Now knead on a lightly floured surface to form a smooth and elastic dough, cover and let rest for 10 minutes. Roll out the dough to a thickness of 1.5 cm. Now cut the donuts out of the dough with a glass about 7 cm in diameter, then - preferably with a finger - poke a hole in the center of the circle. Place the donuts about 7 cm apart on a greased baking sheet and let them rest for 1 hour.
Glaze preparation:
Reduce the cider by half in a saucepan for 10 minutes. In a medium-sized bowl, mix the powdered sugar with the hot cider and the sugar syrup to a smooth glaze.
Let the oil get hot in the deep fryer or in a saucepan (the saucepan should be filled with oil to a height of approx. 5 cm). Now fry 2 to 3 donuts at a time for 2 to 3 minutes, turning the donuts occasionally. Then let it cool down on kitchen paper. Drizzle with the glaze. Serve warm.