In the bowl of an electric mixer fitted with the paddle attachment, beat butter at medium speed until pale and fluffy, 1-2 minutes. Add half of the powdered sugar and beat until well combined, then add the other half and beat until smooth. Add cocoa powder and beat until completely creamy and smooth. Beat in vanilla and melted chocolate until combined. Add heavy cream, a bit at a time, until desired consistency. Add more powdered sugar if necessary, until frosting is spreadable. Add a bit of salt, to taste, to cut sweetness.