Herman cake (often called simply Herman) is a 'friendship cake', similar to the Amish Friendship Bread, for which the ingredients are passed from person to person (like a chain letter) and which continues to grow as it contains yeast and lactic acid bacteria. One starter can, in theory, last indefinitely. The other ingredients for the mixture are milk, sugar, flour and warm water. They became popular in the 1970s.Herman sits on a worktop for ten days and is not refrigerated.