Put the jar in a 100 C oven, for about 30-40 min. Alternatively, place the jar in a pan with very hot water (90 C). The jar should show at this time a clearly separated and transparent oil layer on top (85%) and a watery layer (15%) at the bottom
With a mitt or cloth, carefully take the jar from the oven or water bath
Pour the butter oil layer in a separate container or jar and save for making butter based sauces or for frying. Make sure no watery phase comes along
Then pour the remainder (watery lower layer possibly with some of the butterfat) in a small bowl and place in the refrigerator. When cooled down, the last remaining butterfat can be scooped off when it has solidified. It can be used for fryingThe water phase can be actually consumed. It consists of a slightly viscous, slightly sour milk: this is buttermilk!