Line an 8-inch square baking pan with foil and spray lightly with cooking spray.
Put peanut butter chips and sweetened condensed milk in a large microwave-safe bowl. Microwave in 30 second intervals, stirring well after each until it stirs smooth.
Add peanut butter and stir until incorporated.
Spread peanut butter mixture in prepared pan.
Refrigerate at least 1 hour, then cut into small squares to serve.
Leftovers can be stored at room temperature or in the refrigerator for at least one week.
Notes
We like a nice creamy peanut butter fudge, so we use creamy peanut butter. If you like a little bit of texture, feel free to try crunchy peanut butter.Use a standard peanut butter, not the kind you have to stir before using.