Knead the flour, baking powder, quark, milk, oil and a little salt with a food processor to form a smooth, supple dough. Let the food processor run at medium to high speed for about 5 minutes.
Let the dough rest in the refrigerator for 30 minutes. Roll out the dough on a floured work surface in a rectangular shape (approx. 40 x 45 cm). First cut the dough into 6 rectangles of the same size, cut them in half diagonally.
Roll up triangles from the short side to croissants and bend them slightly. Place the croissants on 2 baking trays lined with baking paper and bake them one after the other in the oven in the oven in front of you.
Remove the croissants and let them cool down.
Meanwhile, for the chocolate mousse filling, roughly chop both couvertures and melt them over a warm water bath. Leave to cool a little. Separate the eggs. Beat the egg yolks, icing sugar and vanillin sugar in a tall beaker with the whisk of the hand mixer until creamy.
Let the couverture slowly run into the egg yolk while stirring.
Separately beat egg whites and cream until stiff. First fold in the cream and then fold in the beaten egg white in portions. ½ Place in a cool place for an hour. Cut the croissants open horizontally. Pour the chocolate mousse into a piping bag with a large star-shaped spout and spray decoratively onto the bases.
Replace the lid.