Pour cold heavy cream into a jar, bowl, or food processor
Whip, shake, or churn the cream until it thickens into whipped cream
Continue whipping, shaking, or churning until the fat separates from the liquid
Strain out the liquid buttermilk
Gather the butter solids together
Rinse the butter under cold water until the water runs clear
Press out any remaining liquid
Add salt if desired
Shape the butter and store it in the refrigerator