Vanilla is the best choice for fudge with the peanut butter flavor front and center, but if you're a fan of a chocolate and peanut butter combo, chocolate frosting will also work.
For a dense and firm consistency, use a non-whipped frosting, and a whipped version for the opposite effect. Use cream cheese frosting if you want a bit of tang.
Begin by adding peanut butter and the frosting into a microwave-safe bowl and heating it in the microwave on high for 30 seconds, stirring when done. Repeat the process once more.
Pour the mixture into a square baking pan and let it chill and set in the fridge for an hour. When done, top it off with a sprinkle of sea salt or your favorite chocolate chips.