In a large mixing bowl, beat the softened butter with an electric mixer on medium speed until creamy and smooth, about 2 minutes.
Add the mascarpone cheese and continue to beat until well combined and fluffy, about 1-2 minutes.
Gradually sift in the powdered sugar and cocoa powder, mixing on low speed until incorporated.
Add the vanilla extract and salt, then increase the mixer speed to medium and beat until the frosting is light and fluffy, about 3-4 minutes.
If the buttercream is too thick, add heavy cream one tablespoon at a time until you reach your desired consistency.
Use immediately to frost cakes or cupcakes, or store in an airtight container in the refrigerator for up to one week.