I've never made yogurt, but was raised churning butter. No, you do not add old butter to the fresh cream. Cream contains two ingredients - butter and buttermilk. We always put the cream into a gallon jar and rocked the jar. The cream should be about 65 degrees F. to make the job go quicker. As the jar is rocked, the butter particles bump together and stick. When the butter has formed into a clump, the job of churning is done. The buttermilk is then stained off and the butter is placed into a bowl. We always "washed" our butter. Run cold water into the bowl and work the butter, drain, and work again. when the water drains off clear, work all the remaining water out of the butter. We then added salt and packaged. This thread is the first I have ever heard of boiling the cream. I would think that if the cow is healthy, the milk handled properly and everything used is clean, there would be no need to boil. To me, that would be just like "store bought" lol.