In a small bowl, whisk together cream and buttermilk. Loosely cover bowl with plastic wrap or a kitchen towel. Let stand at room temperature for 16 to 24 hours.
Once cream has considerably thickened, place in refrigerator to set up completely, about 4 hours. Homemade Crème Fraîche should keep refrigerated in an airtight container for up to 2 weeks.
Recipe by Bake from Scratch at https://www.bakefromscratch.com/guide-homemade-creme-fraiche/