Using an electric mixer fitted with paddle attachment, beat butter at medium speed until creamy.
Add cream cheese and vanilla and beat until fully incorporated.
Add cocoa and mix until blended.
Add melted chocolate and beat until fully incorporated.
Gradually increase mixer speed to high and continue beating until light and fluffy.
Scrape sides and bottom as necessary.
Gradually add confectioners' sugar on low speed until well combined, then beat on high until smooth (about 2 minutes).
If adding heavy cream, add now and beat until desired consistency.
Chef's notes: Melt chocolate chips in the microwave for 30 seconds, remove and stir. Return to microwave and cook at 20 second intervals stirring after each 20 seconds until chips are small pieces melting but lumpy (should not be completely melted). Stir until smooth. Don't overcook, the chocolate will harden. Espresso powder brings out the flavor of the chocolate.