Gather heavy cream (cold), a pinch of salt (optional), and a jar or container with a tight lid
Pour cold heavy cream into the jar/container, filling it no more than halfway
Seal the container tightly
Shake vigorously (or use a mixer) until the cream separates into butter and buttermilk
Continue shaking until butter forms and clumps together, and buttermilk looks mostly clear/yellowish
Stop when butter is grainy and butter solids separate well
Pour off the buttermilk
Add cold water to the butter
Swirl and press the butter to rinse out remaining buttermilk
Drain the water
Repeat rinsing until the water runs mostly clear
Press butter to remove excess water
Add salt if desired and mix briefly
Shape into a block or store in a container
Refrigerate immediately
Use within about 1–2 weeks (salted) or about a week (unsalted)