In a medium saucepan over medium high-heat, combine cream, milk, sugar, cocoa powder and salt. Whisk the mixture until it starts to simmer.
Remove from the heat and whisk in peanut butter until well combined. Add vanilla extract.
Transfer to the refrigerator until completely chilled.
When chilled, add the mixture to an ice cream maker and process according to the manufacturer’s instruction. 5 minutes after churning, add chocolate. If using, add walnuts at the end.
Spoon the mixture into a container and transfer to the freezer until frozen, about 4 hours.
Notes
Footnote: If you are using natural peanut butter, you might want to increase the amount of sugar.