In a small saucepan, combine the coconut milk, sugar, and corn starch. Cook on medium heat while whisking continuously. Cook until the mixture reaches a boil. Once the mixture reaches a boil, turn the heat down to low and simmer for 3-4 minutes.
Remove from the heat and whisk in the peanut butter.
Pour into an empty Ninja Creami pint container and freeze for 24 hours.
After 24 hours, remove the pint from the freezer. Remove the lid.
Place the Ninja Creami pint into the outer bowl. Place the outer bowl with the pint in it, into the Ninja Creami machine and turn until the outer bowl locks into place. Push the Gelato button. During the gelato function, the mixture will mix together and become very creamy.
Once the gelato function has ended, turn the outer bowl and release it from the Ninja Creami machine.
Using a spoon, create a hole through the center of the gelato that goes from top to bottom. This hole is where you will add the mix-ins. Add in the dark chocolate peanut butter cups and chopped peanuts. Be sure to cut the peanut butter cups into small pieces. These mix-ins will not get broken into smaller pieces during the mix-in function so you want to make sure they are in pretty small pieces.
Place the outer bowl with the pint back into the Ninja Creami machine and lock it into place. Choose the MIX-IN function.
Once the Ninja Creami completes the MIX-IN cycle, remove the outer bowl from the machine.