Whip the mascarpone and butter together in a bowl using a hand mixer. Add most of the heavy cream (reserving about a tablespoon) and swerve confectioners (or powdered erythritol).
Then the peanut butter and vanilla extract, mix until smooth and creamy. Add more of the heavy cream if needed, to get your desired consistency.
Add the frosting to a piping bag fitted with the tip of your choice (ziplock bag with the tip cut off will work too) and frost your cooled cake or cupcakes!