How to Make It
Heat cream in a large saucepan over low heat until it reaches 70 degrees. Pour into large bowl and stir in yogurt. Cover tightly with plastic wrap and let sit at room temperature (ideally 72 degrees) for 24 hours. The mixture will thicken slightly and resemble pancake batter. The ambient temperature of the room is critical. Too warm and the milk goes bad. Too cool and the culture won’t grow.
Stir gently and pour into bowl of food processor fitted with metal blade. Process on high speed for 5 to 8 minutes or until small bits of butter form. Strain through a fine-mesh sieve into a large bowl. Let sit 10 minutes, pressing gently on butter with a clean spoon from time to time to encourage draining.
Rinse butter (still in sieve) under cold running water, pressing and squeezing with clean fingertips to help butter form a firmer lump. Fill a large bowl with ice water. Add lump of butter and knead gently to remove last traces of buttermilk, which will make the water turn cloudy. Drain and repeat until water remains clear. This might take 2 to 4 changes of ice water.
For unsalted butter, transfer into clean glass container or wrap tightly in plastic wrap and refrigerate. For salted butter, press butter into a slab on a clean work surface. Sprinkle butter with salt. Fold butter over in thirds as if folding a letter to gently blend in the salt. Taste the butter and add more salt, if desired. Transfer into clean glass container or wrap tightly in plastic wrap and refrigerate.
Pour buttermilk into a clean glass jar, cover tightly, and store refrigerated for up to two weeks.