In a medium bowl, whisk together the cream cheese, sugar, cocoa powder, and peanut butter until smooth.
Slowly whisk in the milk, then add the heavy cream, vanilla, and salt. Whisk until fully combined.
Pour into a Ninja Creami pint container, staying below the max fill line. Cover and freeze on a flat surface for 24 hours.
Remove the lid, place the pint in the outer bowl, attach the paddle lid, and lock into the machine. Select Ice Cream.
If the ice cream is crumbly, select Re-Spin.
Make a hole in the center of the pint with a spoon. Add the chopped Reese's cups and select Mix-In.
Scoop and serve immediately, or freeze 30 to 60 minutes for firmer ice cream.