Whirl cream in a food processor until it separates into buttermilk and clumps of butter-they'll look like fluffy scrambled eggs. Keep whirling until butter forms bigger clumps, about 3 minutes.
Pour mixture into a strainer (set over a bowl if you want to keep the buttermilk) and let drain briefly. Squeeze butter to extract remaining buttermilk (it's okay if a little is left).
Put butter into a second bowl and stir in fine sea salt to taste, if you like.
Note: Nutritional analysis is per Tbsp. butter.
PER 1/2 CUP BUTTERMILK: 75 cal., 20% (15 cal.) from fat; 6 g protein; 5 g fat (2 g sat.); 9 g carbo (0 g fiber); 193 mg sodium; 5 mg chol.