In a microwave-safe bowl, combine the peanut butter and vanilla frosting. Microwave in 30-second intervals, stirring in between, until the mixture is completely melted and smooth. This should take about 1-2 minutes. Be careful not to overheat.
Once the peanut butter and frosting are fully combined and smooth, fold in the chopped peanut butter cups and Reese’s Pieces. Stir gently to distribute the candies evenly throughout the fudge mixture. (Or you can just place the candies on the top instead of stirring them into the fudge.)
Pour the mixture into a greased 8"x8" baking dish. Use a spatula to spread it evenly into the corners and smooth the top. Place the pan in the refrigerator and chill for at least 2 hours, or until the fudge is firm and set.
Once the fudge is fully set, lift it out of the pan using the parchment paper, and cut it into small squares or rectangles. Serve immediately or store in an airtight container.
Store the fudge in an airtight container at room temperature for up to a week or in the refrigerator for 2-3 weeks. You can also freeze it for longer storage, up to 3 months.
Notes
How long does it take for the fudge to set? The fudge typically takes about 2 hours to set in the refrigerator. If you're in a hurry, you can speed up the process by freezing it for 1 hour.