NOTE: It is important that the temperature be consistent. It has been suggested that one puts this in the oven for a steady temp. Just don't forget it's in there!

2 C pasteurized heavy cream 3 T buttermilk

Combine whipping cream and buttermilk in a glass container. Cover and let stand at room temperature (about 70°F) from 8 to 24 hours, or until very thick. Stir well before covering and refrigerate up to 10 days.
Some suggest only partially covering the glass or covering the glass with a breathable material, such as cheesecloth.