In a large bowl, beat the softened butter and peanut butter together until creamy and smooth.
Gradually add the powdered sweetener, mixing well after each addition.
Add the heavy cream and vanilla extract, and beat until the frosting is smooth and fluffy. If the frosting is too thick, you can add an extra tablespoon of cream or milk to reach your desired consistency.
Once the cupcakes are completely cool, spread or pipe the frosting on top of each one.