In a large mixing bowl, beat the softened butter and peanut butter with an electric mixer (wire whisk attachment if using a stand mixer) on medium-high for 1-2 minutes, until smooth, pale, and fluffy.
With the mixer on low, gradually add the powdered sugar and beat until combined.
Add the vanilla, 2 tablespoons of heavy cream, and salt. Beat on medium-high for another 2-3 minutes until light and fluffy. If the frosting is too thick to spread easily, add another tablespoon or two of cream and beat briefly, adding additional liquid only if necessary.
Notes
This recipe will make enough frosting for a two-layer cake or 24 cupcakes. For a pan of brownies or a 9x13 cake, you will want to halve the recipe.