Preparation Whisk cream and buttermilk in a large bowl. Cover loosely with plastic and let sit at room temperature until thickened and tastes tangy, at least 12 hours and up to 36 hours (how long this takes will depend on the temperature of your kitchen). Chill until crème fraîche is cool, about 2 hours. Transfer crème fraîche to the bowl of a stand mixer fitted with the whisk attachment. Cover space between top of bowl and head of mixer with plastic wrap (this is a must, unless you want cream all over your kitchen). Beat on high speed until butterfat has separated and starts to clump around bottom of whisk, about 5 minutes. Line a fine-mesh sieve with cheesecloth. Strain butter mixture through sieve into a small bowl. Pull cheesecloth tightly over butter and form into a ball. Squeeze to drain any excess buttermilk; reserve buttermilk for another use. Transfer butter to a clean bowl and knead, adding a few tablespoons of ice water at a time. Drain water off butter as it becomes cloudy; repeat 3–4 times (the water you drain off should become clearer each time). Continue to knead until no liquid remains. Transfer butter to an airtight container or roll into parchment-wrapped logs. Wrap in plastic and chill until ready to use. Serve with sea salt, if desired. Do Ahead Butter can be made 2 weeks ahead; keep chilled, or freeze up to 3 months.