In a clean jar or bowl, combine the heavy cream and buttermilk or plain yogurt. Mix well. Cover loosely with a lid or cloth. Allow the mixture to sit at room temperature (around 70°F or 21°C) for 24-48 hours until it thickens and develops a tangy flavor. Stir the sour cream, then store it in the refrigerator for up to 2 weeks. Before using, give it a stir to achieve the desired creaminess.