Pour cream into the bowl of a stand mixer using the whisk attachment, or use a mixing bowl and hand mixer. Begin beating the cream on a low speed, and gradually increase the speed as the mixture thickens.
After several minutes, whipped cream will form. Continue mixing. The whipped cream will start to yellow. After several minutes more the fat solids will separate from the liquids and you'll see clumps of butter fat quickly form, leaving the sloshing liquid buttermilk in the bottom of the bowl.
Mason Jar Instructions
Pour heavy cream into the mason jar, filling it half-way full. Do not overfill past the half-way mark. Screw the lid on tightly so there are no leaks.
Shake mason jar for approximately 5-7 minutes. After a few minutes, whipped cream will form. Keep shaking until you hear that a lump has formed inside, and shake an additional 30-60 seconds after that. You should clearly see that the fat solids have separated from the liquids. The amount of time it takes will depend on how vigorously you are able to shake the jar.
Rinsing, Flavoring, and Storing
Strain the fat solids from the buttermilk using a fine mesh strainer or cheesecloth. You can reserve the buttermilk and save it for later use in recipes.
Pour cold water over the butter. Discard water and repeat rinsing 2 times more. Shape into a ball or any other shape (you can purchase butter molds for interesting designs).
At this point you have butter. You can add in things like salt, honey, and herbs to create flavored butters, or serve in its pure form as is.