Pour the buttermilk into the slow cooker. Cover and turn the cooker to the warm or keep-warm setting (or the yogurt setting at "normal" on an Instant Pot).

The buttermilk will separate into a creamy top layer about the thickness of Greek yogurt (but fluffier in texture) and a watery bottom layer and will be firm and just warm to the touch in the center. This will take 2 to 8 hours, depending on your cooker.

Put a sieve or colander over a bowl and line it with four layers of rinsed and squeezed cheesecloth and use a slotted spoon or skimmer to gently spoon the creamy top layer in, leaving the watery whey behind.

Let drain at room temperature for 2 to 4 hours, then transfer to a sealable container and refrigerate for up to 1 week.