Homemade Crème Fraîche (Sour Cream)
multiplies with no known issues
1 cup heavy whipping cream
1 Tbsp buttermilk
[Optional step 1: pour cream into a saucepan and heat to 105˚F. I generally don't do it.]
Combine cream and buttermilk in a jar, screw on the lid, and shake it around for about a minute. Leave it on the counter, in a warm place and undisturbed, for 24 hours. It should thicken and become tangy. If you leave it longer, it will get thicker and tangier. Put it in the refrigerator for 12-24 more hours. My last batch turned out as thick as store bought sour cream, but the one before that only hit sloppy dollop stage. Heat+patience=better crème fraîche.
Stores in the refrigerator, tightly covered for 3-4 weeks.