After a lot of searching I have found this recipe that I think explains canning butter really well and safely. As its a non-acidic product it needs to be pressure canned rather than water canning. If you water can you will run the risk of botulism, so please pressure can to be safe.
Canned butter will last for years as long as the seals are still ok and make sure you keep in a cool place.
Its best if you use wide mouth, half pint canning jars but as the author of this recipe says, she has used half pint jelly jars. The wide mouth jars are just easier to get the butter out of.
This recipe takes 5 pounds of butter and you get around 10 half pints from it.
Prepare your jars and put 3-4 inches of water in your pressure canner and bring up the heat.
Put the butter in a pot and melt it, bring it to a boil then reduce the heat to a simmer. Let the butter simmer for 10 minutes or so. You’re evaporating some of the moisture from the butter and getting it good and hot. Be sure to stir it and don’t allow any of the milk solids ( the little while particles) to burn and don’t let it boil over!
Once everything is good and hot, carefully take the pan of jars out of the oven. Place the jars right side up on a clean dish cloth. Insert the plastic funnel in the jars one at a time and fill the jars with hot butter, leaving a 1/2″ head space.
Using a clean cloth dipped in vinegar and then squeezed out, clean off the jar rims.
Place lids and rings on each jar, tighten the rings just firmly but not hard.
Put the jars in the pressure canner that has 3-4 inches of hot water in it.
Put the lid on the canner and fasten it down. Allow steam to flow out of the vent for about 10 minutes. Place the weight (at 10 pounds) or gauge (11 pounds) on the vent and when it reaches the correct pressure (when the weight begins to rattle) start timing.
Allow the pressure canner to chug along for 60 minutes for these half pints.
After 60 minutes, take the canner of the heat and allow it to release its pressure naturally. You might hear the lids sealing while the jars are still in the canner, that’s OK.
Once all the pressure is gone and you can safely open the canner, open it up and take the jars out of the canner. Place them on a clean towel to cool.
Because this is pressure canned, and the butter is subjected to high temperatures, you may see the milk solids clump together and fall to the bottom of the jar. That’s OK! Jut give your jars a shake as they cool to combine the solids and the liquids. As the butter solidifies, the solids will be dispersed throughout the liquids and it will look like you expect butter to look.
Check for seals after it cools. Store canned butter in a dark cool place. If you want the butter to be firm, then refrigerate a jar before you open it.
This recipe via (and to read more) please go here: christianhomekeeper.org