Put a cup of heavy cream in a saucepan and mix 1 tablespoon of buttermilk in it. 

Gently heat this mixture until it is somewhat warm and then transfer it into a glass container or bowl. (Don’t let it get hot. We just want to warm it.)

Use a clean dishcloth to cover the bowl and let it sit at room temperature for at least a day.

Stir the prepared crème fraîche mixture and put it in your fridge until it’s chilled.