Combine the three ingredients in a wide pan (I used a 5 1/2 quart Dutch oven) and bring to a simmer. Reduce the temperature to low and let it gently bubble, stirring every five minutes or so. If it’s sticking to the bottom of the pot badly, lower the heat a bit more. Cook like this for at least an hour, until butter has reduced in volume and has turned a deep, rosy color.
Prepare one pint or two half pint jars. When butter is sufficiently cooked down, ladle it into jars, leaving 1/2 inch headspace. Wipe rims, apply lids and screw on bands. Process in a boiling water canner for fifteen minutes.
When processing time has elapsed, remove jars from canner and place them on a towel-lined countertop. When jars have cooled enough to handle, remove the rings and test the seals by gently grasping the lids and lifting the jars. If the lids hold fast, your seals are good. If your jars do not seal, store product in refrigerator and consume within a month.
Sealed jars can be stored in a cool, dry place for up to six months.