Beat softened butter and cream cheese together on medium high speed until fully combined (about 2 minutes). Scrape down the bowl.
Add powdered sugar, cocoa powder, salt, vanilla, heavy cream, and milk. Start mixer on low for 30 seconds. Bump speed to medium and beat for 2 minutes. Scrape down the bowl, taste, and add more salt if needed.
If frosting is too thick, add 1 tbsp milk. If frosting is too thin, add 1/4 cup powdered sugar.