Combine the butter and cream cheese in a large bowl and, using an electric mixer or a stand mixer to beat until creamy, well-combined, and lump-free.
Add vanilla extract and salt and stir well to combine.
With mixer on low, gradually add powdered sugar until completely combined.
Add cocoa powder and beat until well-incorporated into the frosting -- be sure to scrape the sides and bottom of the bowl so that all ingredients are well-combined.
Frost completely cooled cake or cupcakes.
*If you plan to pipe the frosting with a small tip, I recommend sifting the powdered sugar and cocoa powder (after measuring) to remove any lumps. The cupcakes in these photos were frosted using an Ateco 848 tip.