Sterilize a glass jar by washing it with hot, soapy water and rinsing thoroughly.

Pour the heavy cream into the sterilized jar.

Add the buttermilk to the jar and stir gently to combine.

Cover the jar with a clean cloth or paper towel and secure it with a rubber band.

Let the mixture sit at room temperature (around 70°F) for 24 hours.

After 24 hours, stir the mixture. It should be thick and tangy. Add salt if desired.

Cover the jar with a lid and refrigerate for at least 2 hours before using.