Saw this on I'm a baker site and think while not so common as the vinegar method, makes for a tart and creamy buttermilk. The cook time is waiting period and serving size is certainly a guess depending on how you use it.
Serving Size: 1 (59) g
Servings Per Recipe: 4
Calories: 74.1
Calories from Fat 59 g 81 %
Total Fat 6.7 g 10 %
Saturated Fat 3.9 g 19 %
Cholesterol 18 mg 6 %
Sodium 36.1 mg 1 %
Total Carbohydrate 2.3 g 0 %
Dietary Fiber 0 g 0 %
Sugars 2.5 g 10 %
Protein 1.6 g 3 %