Beat the whipped cream and vanilla sugar with the whisk for about 6 - 7 minutes, until the buttermilk separates from the fat and a nice lump of butter has formed. Then put it in a cloth and let it drain off.
Shape into a roll or ball and store in the refrigerator. Goes well with raisin mares or sweet yeast rolls. Do not throw away the buttermilk, it is also very tasty.