Easy and delicious. Per Serious Eats, crème fraîche is soured cream, or cream that has been thickened and acidified by the growth of bacterial cultures. Often sold at specialty markets, crème fraîche can be used in a number of ways in the kitchen. For example, you can serve crème fraîche alongside fresh fruit, or you can stir crème fraîche into soups or sauces to add a little bit of dairy richness and thickness; it also contributes a bright bit of acidity to any dish.
Combine buttermilk and heavy cream in non-reactive container. Cover and allow to rest at room temperature until thickened to desired texture, about 12 hours. Store in the refrigerator for up to 2 weeks.
Note:
The bacterial cultures in buttermilk thicken and acidify the cream, while also preventing bad bacteria from taking over.