I ran across this in an archived Y2K site. What do you think?
HOW TO CAN BUTTER:
1. Use any butter that is on sale. {Lesser quality butter requires more shaking (see #5 below), but still turns out beautiful--and costs far less.}
2. Heat pint jars in 250 degree oven for 20 minutes, without rings or seals. {One pound of butter is a little more than one pint jar, so if you melt 10 pounds of butter, heat 11 pint jars.}
3. While jars heat, melt butter slowly until it comes to a boil. Reduce heat, cover and simmer for 5 minutes. {Also put canning lids in pot of water and simmer for 5 minutes.}
4. Pour melted butter carefully into heated jars using a large ladle, stirring the melted butter before pouring into each jar. Use a canning jar funnel, then wipe the rim of the jar with a damp towel. Leave 1/2" to 3/4" air space, which makes shaking easier.
5. Add lid and ring and close securely. They will seal as they cool. Shake while the jars are still warm but cool enough to handle easily, because the butter separates and becomes white on the bottom of the jar. Shake again, and as the jars cool, shake again.
6. At this point, while still slightly warm, put the jars into a refrigerator. While hardening, shake again, and the butter will then look like butter and be firm. This final shaking is very important! Check every 5 minutes and give a little shake until hardened in the jar! Leave in the refrigerator for an hour.
7. Canned butter should store for 3 years or longer on a cool, dark shelf.