In a jar or other glass container, combine the cream and buttermilk. Cover loosely and let stand at room temperature (about 70 degrees) for 12 to 24 hours until very thick and the mixture resembles the texture/thickness of thick yogurt. Stir well, cover and refrigerate for up to 2 weeks.
For the focaccia, I used this homemade focaccia recipe, split it in half to form two thinner pieces of focaccia, mixed about 1/2 cup creme fraiche with 2 tablespoons pesto and smeared that mixture on the cut sides of each piece of focaccia. Then I layered on cherry tomatoes, split in half, and fresh mozzarella cheese - broiled it for about 4-5 minutes (watching carefully!) and devoured nearly singlehandedly.
Notes
Whipping Cream: the key to homemade crème fraiche is NOT using ultra-pasteurized whipping cream. If you have the Dean's brand of cream in your grocery stores, I've found it is one of the few that is just pasteurized, not ultra-pasteurized so it will thicken beautifully. I'm sure there are other pasteurized brands also...so look closely at the cartons to make sure you know what you are buying.