Ingredients:

1 cup whipping cream*
1 Tbsp. Buttermilk

*Note: do not use ultra-pasteurized cream - it does not have enough bacteria to curdle.

Methods:

1. Heat whipping cream in a small saucepan to 95°. Transfer cream to a clean jar with a tight fitting lid. Add buttermilk, shake well. Let stand at room temperature until thickened (12-24 hours). Refrigerate.
2. For a quicker version, you can substitute 1 cup of sour cream and whisk in the juice of one lemon.