This topic is a huge controversy among canners. I assure you these facts are accurate. Many get offended (that is not my intent) that they are told they can’t do what their grandparents did safely in this day and age. Facts are facts and I hope this clears it up for you.
“Botulism spores are killed after being exposed to temperatures of 240˚ for a specific period of time. Boiling water only reaches 212 degrees. Water boils at 212° F near sea level and because of the heating curve even if you increase the temperature on your stove the water NEVER gets any hotter, so water bathing your butter also will not bring it up to a temperature where Clostridium botulinum can be killed. Butter is a dairy product and shouldn’t be canned. Milk and butter are “low acid foods”, this means they can support the outgrowth of C. botulinum and toxin formation in a sealed jar at room temperature. Unique fats in milk and butter (the dairy fats) can protect botulism spores from the heat of even a pressure canner. SO, if you buy butter from the store and there was one person at the dairy where that butter came from, that was not following cleanliness procedures, you could get sick from canning it. Botulism spores (commonly found in dirt) are not killed by the pasteurization process of milk so they can still be in there at any time. While undesired, this usually isn’t a big deal because if ingested they can pass through a healthy adult without causing any harm (or a resulting product recall). However, if you take that same dairy product containing the spores and put in a jar where you seal it off from oxygen for an extended period of time at room temperature…..it can grow undisturbed into one of the many strains of Clostridium botulinum, and produce an extremely lethal neurotoxin until you open it up and eat it.”