In a large mixing bowl, combine the unsalted and salted butters. Using an electric mixer, beat them together on medium speed until the mixture is light and fluffy, about 2-3 minutes.
Gradually add the powdered sugar and cocoa powder, mixing on low speed until just incorporated to avoid a sugary cloud.
Add the heavy cream and vanilla extract, increasing the mixer speed to medium-high. Beat until the mixture is smooth and fluffy.
Taste your frosting and adjust the sweetness or chocolate flavor as desired by adding more powdered sugar or cocoa powder.
Use a spatula or piping bag to frost your cakes or cupcakes and enjoy!
