Fill jar 1/2 to 3/4 full with cream. Place lid on tightly. Shake for ten to thirty minutes, or until you hear solids shaking around and can see through the glass that the solids have separated from liquid, the liquid is going to completely coat the jar as you shake and you’ll hear it, but as you continue to shake you won’t really hear it
After about fifteen or twenty minutes, you’ll notice the sides are no longer coated and it pulls away from the sides a bit.
Then a few minutes later you’ll feel solids shaking around in there and the liquid and solids will separate completely. Pour into colander to separate liquid from butter. Place butter in a ramekin or bowl and stir in salt, if desired. Refrigerate until hardened.