Fill the mason jar half way with cold heavy cream. Fasten the lid tightly and shake shake shake. Shake until your arms get tired, then continue shaking until they want to fall off, and then shake some more. About 25 minutes total. After about 7 minutes of shaking you'll notice that the cream starts to thickly coat the outside of the jar, and after about 15 minutes you'll see the buttermilk start to separate from the butter.
Once the butter is completely separated from the buttermilk, you can pour the contents of the jar through a strainer to get all of the buttermilk out. The butter will be extremely soft at this point, but will harden up once you put it in the fridge.
I just love the color of this butter, a very faint yellow as opposed to the fake bright yellow you find in most grocery store butters. You can add a little bit of salt at this point to the butter if you prefer the salted variety, but I just kept it as is.
And this is what you end up with. I used a pint of creme and ended up with 1 cup of butter milk, and 1 cup of butter. Buttermilk can easily be frozen and used at a later date. While I haven't calculated if this is a cost effective way get butter, it sure was fun! And honestly, who can sit down at lunch with their work friends and say "guess what I did this weekend, made butter". Not many! Hope y'all have a great week!