What you will need:
Butter from grass-fed cows, salted or unsalted (2-8 bricks, depending on how much you want to make)
Nut Milk Bag or Doubled-Over Cheesecloth
Strainer (optional: mostly just to make things easier than wrangling the nut bag or cheesecloth)
Please! Do not scrimp on the butter being from grass-fed cows--I've tried this with butter that was not from grass-fed cows and trust me, you do not want to do this. With the salted butter, a lot (but not all) of the salt mixes in with the milk solids and gets strained out.
Let's Get Started
Place at least two "bricks" (totaling 1 lb or more) butter from grass-fed cows that is unsalted into your crock pot and set to Low. I use 2 bricks, as the crockpot I use is a small one that is 2 quarts. I've made it with 3-4 bricks in my small crockpot. If you have a larger crockpot, you can use anywhere from 4-8 bricks. Just make sure you have a container large enough to store it in once you're done!
Set it on high for about 45 minutes, then low for 8-12 hours. Lid ON.
How Do I Know When It's Done?
You will notice the clear butter (or ghee) is sitting on top of the milk solids (the milk solids sink to the bottom of your crockpot). That's the way it should be -- it's just the opposite of how it works on the stovetop.
Straining the Butter To Get The Milk Solids Out
Put a Nut Milk Bag or doubled over Cheesecloth over the container you'll store it in and pour your entire mixture through it to strain out the milk solids. You should end up with almost as much product as you started out with. There should only be a small amount of milk solids left that can be trashed.
This photo shows a small amount of the milk solids in the Nut Milk Bag that I used --
you can just toss these out once you're done.
The product in your storage container will be completely liquid -- that's the way it should be. If you want to firm it up a bit, put it in your refrigerator for a few hours -- but long term, there is no need to refrigerate it.
The photo below is actually more translucent/see-through than it appears to be.