Night before: Proof your starter. I usually get mine out of the refrigerator, where I store it, and add a cup of flour and a cup of water. Then I let the mixture sit loosely covered overnight on the counter. It should be bubbly and active in the morning.
Heat the milk and the butter in the microwave for one minute. Allow to cool slightly. Add yeast and let sit until yeast dissolves.
In a large bowl, mix the proofed sourdough, egg, salt and sugar. Add the milk mixture. Gradually add the flour to make an elastic dough.
Knead for 5 minutes.
Let the dough rest for 10-15 minutes.
Roll out the dough on a lightly floured surface to approximately 9" x 13".
Sprinkle the surface of the dough with the cinnamon mixture (and raisins if you are adding them).
Roll up the dough.
Lightly score the roll to create 12 rounds. Cut firmly with a sharp knife.
Put the rounds in a lightly buttered 13"x9" pan.
Allow to rise until rounds are touching each other and the sides of the pan. Approximately 60 minutes in a warm place.
Bake at 350 for 15-20 minutes.
Mix the icing to your desired consistency. Some people like it thicker, some prefer thin. Pour over the cinnamon roles while hot.