In a food processor, combine the peanuts, vanilla extract, and salt. Process until the mixture becomes peanut butter. It doesn't have to be perfectly smooth. Add the cocoa and process until well combined.
You can use store-bought peanut butter if you prefer. Either buy chocolate peanut butter or make your own by stirring some cocoa in (this may work best in a food processor). You may need to to add more powdered sugar if you go this route, but just mix until you get the consistency you need to form balls and you're good.