Prepare an 8x8 or 9x9 baking dish by lining it with waxed paper.
In a large microwave-safe bowl, add the butter and peanut butter. Heat in the microwave on high for 1 minute or until the butter has melted. Stir to combine.
Add the marshmallow cream and powdered sugar to the peanut butter mixture, stirring until blended. The mixture will be very thick.
Transfer the fudge into the prepared dish and smooth into an even layer.
Refrigerate for at least 2 hours prior to slicing and serving.
Store in an airtight container in the refrigerator for up to 2 weeks.