Yeast reproduces asexually by budding. This means that each yeast starter is a clone, and each cake will be a clone. Cloned German cakes, spreading through the population, numbers increasing at an exponential rate - what could go wrong?
A swift google suggests that the canonical form of this living chain letter is Amish Friendship Bread. In fact, the article notes that there is a 2011 novel of such a bread spreading through a town and
 This would be literally exponential, not metaphorically.