Makes about 1 2/3 cups crème fraiche.
I find that buttermilk makes a better tasting crème fraiche with a firmer set than lemon juice. I also find that letting the crème fraiche ripen in a plastic container works better than glass or stainless steel.
Stir buttermilk into whipping cream and pour into a plastic container like a juice pitcher and cover with plastic wrap. Place pitcher in a larger container, and pour hot tap water around the pitcher, filling up to the height of the cream. Place container in a warm, draught-free spot (I find on top of the fridge is best) and let sit for 24 to 48 hours (it ripens faster in summer than winter).
To determine if set, jiggle the pitcher slightly – the cream should be not shift and should have sweet-lemony smell. Chill completely. Spoon off crème fraiche, being certain not to stir in the liquid whey, which will settle at the bottom of the pitcher.