Herman cake, also just Herman or Herman friendship cake, is a sourdough cake whose starter is passed from person to person. It is mostly associated with Germany, where the practice appears to have developed in the 1970s, possibly during the German peace and ecology movements. In Germany, the baked good is called Hermann, and also Glückskuchen ("luck cake"), Glücksbrot ("luck bread"), or Vatikanbrot ("Vatican bread").
The custom of passing a sourdough starter to friends bears a strong resemblance to the American Amish practice of Amish friendship bread; given the cultural exchange between the American Amish, Pennsylvania Dutch, and German communities a connection is very likely.