1. Heat cream, milk, and peanuts in a medium saucepan over medium heat until bubbles begin to form around edges. Remove from heat, cover, and let steep at room temperature for 3 hours (or refrigerate overnight).

2. Prepare an ice-water bath. Uncover cream mixture, and reheat until hot but not boiling. Whisk yolks, sugar, and salt in a large bowl. Add cream mixture in a slow, steady stream, whisking constantly until well combined. Return mixture to saucepan, and stir constantly over medium heat until thick enough to coat the back of a spoon, about 8 minutes. (Do not let the mixture come to a simmer.)

3. Strain through a fine sieve into a bowl. Discard solids. Set the bowl in the ice-water bath. Stir in vanilla, and let cool, stirring occasionally, until cold, about 20 minutes.

4. Freeze mixture in an ice cream maker according to manufacturer's directions. Transfer to a large bowl, and fold in peanut butter until ice cream is swirled. Use immediately, or freeze for up to 1 week.