Line an 8-inch square pan with waxed paper or parchment; set aside.
Spoon peanut butter into a medium or large microwave-safe bowl and heat in the microwave on high for 30 seconds. Stir.
Add cream cheese frosting and stir until smooth and thoroughly combined.
Spread mixture into the prepared baking pan. Refrigerate until chilled, about 1 hour before cutting into 16 squares.
Store in an airtight container in the fridge for up to 1 week.
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Notes
To Make Peanut Butter Chocolate Fudge
Use chocolate frosting instead of vanilla frosting.To MakePeanut Butter Cup Fudge
Make two-ingredient peanut butter fudge recipe as directed and let set in the fridge for an hour or two.While fudge is cooling, make chocolate ganache by heating 6 ounces whipping cream until is it hot, but not boiling. Remove from heat and add 8 ounces dark chocolate chips (or semi-sweet if you prefer). Do not stir. Let sit for 2-3 minutes, then stir until the chocolate is melted and smooth.Let the ganache cool for 10-15 minutes, then spread over fudge. Return to fridge to cool completely.To Make Layered 2 Ingredient Chocolate Peanut Butter Fudge
Make two-ingredient peanut butter fudge recipe as directed.Make a second batch made with the same amount of peanut butter and chocolate frosting in place of vanilla frosting.Spread the peanut butter fudge into a 9x13" baking dish, then spread the chocolate peanut butter fudge on top. Refrigerate a few hours until set.