In a medium saucepan, warm the heavy whipping cream over low heat until it reaches 105 degrees F. Remove from heat and stir in buttermilk. Transfer to a small bowl and cover with plastic wrap. Let stand in a warm place until thickened, 24 to 36 hours. Stir every 6 hours. It will be ready when it has a nutty taste. Chill cream in the refrigerator for 2 hours before use. Store in the refrigerator for up to 10 days.