The traditional coating of the mold is usually done with melted beeswax - especially if using copper molds. I have tried both soft butter and beeswax with my aluminium pan. I found beeswax to be very hard to use as it hardens extremely quickly, so you have to work very quickly. It is also quite hard (or impossible) to clean. Butter is obviously much easier to find, and also way less messy. There was no real difference in taste or texture in the batches made using beeswax or butter so I personally recommend using soft butter for ease.