Mix the quark with sugar, eggs, milk and baking soda. Sift the flour and stir in. Peel and core the apples, cut into small cubes and mix with the batter.
Cut off with a teaspoon donut and let slide into the hot fat. The fat in the deep fryer should not be hotter than 190 °, as the donuts should have a light brown color on the outside but must also be done. Roll it in the cinnamon-sugar mixture while it is still hot.
A relaxed, airy, delicious thing. Makes about 20 donuts.