Here, I've found a bit more info about it:
What Is This German Friendship Cake?
Herman the German friendship cake is a sourdough cake recipe which is several decades old. Obviously the mix isn't decades old but you get the idea! It passes through circles of friends and family similar to a chain letter.
The origin may have been from the Amish people, who used something similar - sourdough bread that was passed around to the sick and needy. If you know where this idea originally came from please feel free to comment.
The Herman Starter Mix
If you cant wait to receive a starter for this great little cake, the starter mix is very easy. Here's how to start your own.
Ingredients:
5oz plain flour
8oz castor sugar
Half a pint of warm milk
2 fl oz lukewarm water
1 packet of active dry yeast
Instructions:
Dissolve the yeast in warm water for 10 minutes then stir.
Add the flour and sugar then mix thoroughly.
Slowly stir in the warm milk.
Cover the bowl with a clean cloth.
Leave in a cool, dry place for 24 hours.
Now begin at day one of the 10 day cycle, as per the friendship cake instructions.
A bubbling dish of goo, sorry - 'German friendship cake mix' (flour, milk and sugar apparently). The instructions I received were quite clear 'You cannot put me in the fridge or I will die. If I stop bubbling, I am dead.' What a responsibility I had ahead of me.
I read on and felt relieved as the only thing you really have to do from day to day is give him a stir.
Half way through the ten day cycle add some flour, milk and sugar then on the tenth day add the ingredients for the cake and bake. How difficult could this be?
A common cycle is based on the addition of one cup each of sugar, flour, and milk every five days, with the mix being shared into equal portions and the cake being baked every tenth day. The ten day cycle produces four starter mixes, which must be either used to bake a cake, given away, or used to start a new cycle. It is common to bake one part and give away the other three.
German Friendship Cake Instructions
Here are the instructions I received with my little Herman. Quite straight forward really.
Hello, my name is Herman.
I am a sourdough cake. I'm supposed to sit on your worktop for 10 days without a lid on.
You CANNOT put me in the fridge or I will die. If I stop bubbling, I am dead.
Day
You get Herman and put him in a large mixing bowl and cover loosely with a tea towel.
Stir well
Stir well
Herman is hungry. Add 1 cup each of plain flour, sugar and milk. Stir well.
Stir well
Stir well
Stir well
Stir well
Hungry again. Add the same as day 4 and stir well. Divide into 4 equal portions and give away to friends with a copy of these instructions. Keep the 4th one.
Herman is very hungry. Stir well and add the following:
1 cup sugar
half tsp (teaspoon) salt
2 cups plain flour
2/3 (two thirds) cup of cooking oil
2 eggs
2 tsp vanilla essence
2 cooking apples cut into chunks
1 cup raisins
2 heaped tsp cinnamon
2 heaped tsp baking powder
Mix everything together and put into a large greased baking tin. Sprinkle with a quarter of a cup of brown sugar and a quarter of a cup of melted butter. Bake for 45 minutes at 170-180C. When cold cut into finger pieces. Cake freezes well and is also delicious warm with cream or ice-cream.
Never had one myself but I know friends who had 
