Pour heavy cream into a clean jar, bowl, or mixer
Let the cream sit at room temperature for 15 to 30 minutes
Shake the jar vigorously or beat the cream with a mixer
Continue until the cream thickens into whipped cream
Keep shaking or beating until the fat separates from the liquid
Strain out the liquid buttermilk
Rinse the butter under cold water
Press and knead the butter to remove remaining buttermilk
Add salt if desired
Shape the butter and store it in the refrigerator