In a clean glass jar, gently whisk together the heavy cream and buttermilk until just combined. Ensure the buttermilk is well incorporated but avoid over-whisking.

Cover the jar loosely with a lid or cheesecloth secured with a rubber band. This allows for air circulation while keeping contaminants out.

Leave the jar at room temperature (ideally between 70-75°F or 21-24°C) for 12 to 24 hours. The time will vary depending on the ambient temperature. You'll know it's ready when the cream has thickened considerably and has a pleasant, tangy aroma. It should be thick enough to coat the back of a spoon.

Once thickened, give the crème fraîche a gentle stir. Seal the jar tightly with the lid and refrigerate for at least 6-8 hours, or preferably overnight. This chilling period is crucial for allowing the flavors to meld and the texture to fully develop.

Your homemade crème fraîche is now ready to use! Stir gently before serving.