Use heavy cream or full-fat milk
Chill the milk or cream well
Pour it into a jar, bowl, or mixer
Shake, whisk, or churn until the fat separates from the liquid
Continue until yellow butter solids form and buttermilk remains
Drain off the liquid buttermilk
Rinse the butter under cold water
Press or knead out any remaining liquid
Add salt if desired
Shape and store the butter in the refrigerator