Add heavy cream and sea salt (if using) to quart sized mason jar.
Replace mason jar lid and ring and tighten to finger tight. Shake 8-10 minutes until fats separate into homemade butter.
Pour into fine mesh sieve and allow to drain.
Save buttermilk.
Rinse butter until all buttermilk residue is removed. Use cheesecloth to remove water residue. Store in refrigerator or freeze.
Notes
Homemade butter will keep refrigerated for 7-10 days depending on how much buttermilk and water residue is left in it. If frozen, it will keep up to 3 months.