Pour cream into the jar and screw on the lid. Shake jar until butter forms a soft lump, 15 to 20 minutes. Continue to shake until buttermilk separates out of the lump and the jar contains a solid lump of butter and liquid buttermilk.
Pour contents of the jar into a fine mesh strainer and strain out the buttermilk, leaving the solid butter.
Remove the lump of butter and wrap in plastic wrap. Refrigerate until needed.