Stir one cup of heavy cream with 1 tablespoon of cultured buttermilk in a clean glass bowl or mason jar. Cover with lid or cheesecloth.
Let sit at room temperature (around 72° F) for 24 hours.
Once it is thick and tangy, it is done. Store in the fridge for a couple of weeks.
Notes
Keep in mind this creme fraiche substitute needs to sit for 24 hours to thicken!
How to Store
Refrigerate - After your homemade creme fraiche substitute has sat out to thicken, you need to refrigerate it. It can be stored in an airtight container for up to two weeks.
Freezer - You can freeze store bought creme fraiche and this alternative as well. However, keep in mind that like all dairy products, it might separate a little bit - this doesn't mean it's gone bad, but the consistency won't be quite as creamy. Store it in an airtight container for up to three months in the freezer.