Knead a dough from the flour, the butter at room temperature, the Parmesan and the sour cream, add just a pinch of salt, as the cheese is already salty, and a little black pepper. Shape the dough into a roll (3-4 cm in diameter) and place in a freezer bag in the refrigerator for 1/2 hour.
Cut approx. 1/2 cm thick slices from the roll, place on a baking tray lined with baking paper, brush with egg white and top with rosemary needles, preferably fresh.
Bake in the oven at approx. 200 degrees for 15 minutes until they are very lightly browned.