Pour heavy cream into the mason jar, filling it half-way full. Screw the lid on.
Shake mason jar for approximately 5-7 minutes. After the first 2 minutes you'll have whipped cream. Keep shaking until you hear that a lump has formed inside, and shake an additional 30-60 seconds after that.
Remove the solids from the jar. The remaining liquid is buttermilk. You can save that for other recipes, or discard it.
Place the solids into a small bowl. Pour cold water over the butter and use your hands to squish it into a ball. Discard water and repeat rinsing 2 times more.
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.