Mix the flour, salt, sugar and yeast in a bowl. Add the butter, quark and milk, mix, knead into a smooth dough using the dough hook on a food processor for approx. 5 mins. Divide the dough into 12 portions, shape into buns, place on a sheet of baking paper, cover and leave to rise at room temperature for approx. 1½ hrs.
In a pan, whisk the milk and all the other ingredients up to and including the egg yolk, bring to just below the boil over a medium heat, stirring constantly. Remove the pan from the heat immediately. Stir the gelatine into the hot custard, then pour the mixture through a sieve into a bowl. Leave the custard to cool, cover and chill for approx. 1 hr. until the custard is just set at the edges. Stir the custard until smooth. Carefully fold in the quark and whipped cream. Transfer the custard to a piping bag with a smooth nozzle (approx. 6 mm in diameter), chill for approx. 1 hr.
To deep fry
Fill a pot to 1/3 with oil and heat it to approx. 160°C. Flatten the dough balls slightly. Using a slotted spoon, slide them into the oil in batches (bottom side up), deep-fry for approx. 2 mins. on each side. Remove and drain on paper towels.
Pour the sugar into a deep dish and roll the doughnuts in the sugar. Insert a straw two-thirds of the way into the doughnuts, work the straw backwards and forwards a few times until the hole is approx. 5 mm wide. Pipe the filling into the doughnuts.