Add the quark, cooking oil, sugar, egg and salt to the mixing bowl and whisk. Mix the baking powder into the flour, add it little by little and knead it with a handheld mixer (dough hook) at a slow speed for roughly one minute until a smooth dough forms. Add the milk at the end.
Cover the wholemeal quark oil dough and leave to rest in the fridge for 15 minutes.
Nut filling: Lightly beat the egg white and sugar with a handheld mixer, but not until stiff. Stir in the hazelnuts and cream. If you like, feel free to add 2 g of cocoa.
Preheat the oven to 175 °C (convection).
Roll out the dough on a work surface to about 40 x 40 cm and cut out rectangles 10 cm wide and 8 cm deep. Portion a teaspoon of nut mixture (10 g) on top. Dilute the egg yolk with a little milk and brush the edges of the dough with it. Place the top side of the pastry over the filling and press down slightly.
Place the hazelnut cookies on two baking trays lined with baking paper and brush the surface with the remaining egg wash.
Bake each tray on the middle rack for about 18 – 20 minutes.
The dough can also be filled with marzipan, for example.