Melt the chocolate in the microwave or in a double boiler. Let cool until it feels just barely warm.
Beat butter until light and creamy. Drizzle in cooled melted chocolate and beat well. Add powdered sugar, cocoa, and salt and mix until well combined. Spoon in vanilla and cream/milk, and beat on high for about 30 seconds.
Spread or pipe onto cooled cake or cupcakes, and enjoy!