Warm the peanut butter. Microwave the peanut butter 20–30 seconds until pourable. Whisk in the sugar, cocoa powder, and salt until smooth.
Make the base. Slowly whisk in the milk, heavy cream, and vanilla until fully combined and lump-free.
Freeze. Pour into the Ninja Creami pint (below the max line). Lid on, freeze flat and level for at least 24 hours.
Spin. Remove lid, install in the outer bowl, lock in, and run the Ice Cream function.
Re-spin. If crumbly, run Re-Spin 1–2 times until smooth and creamy.
Add mix-ins. Make a 1½-inch hole to the bottom of the pint, add peanut butter cups and chocolate chips, and run the Mix-In function.
Serve. Scoop immediately for soft-serve, or refreeze 15–20 minutes for a firmer scoop.