Two kinds of chocolate—chips and cocoa powder—make this frosting extra chocolaty!
Place chocolate chips in medium microwave-safe bowl. Heat in microwave at 50 percent power for 1 minute. Use rubber spatula to stir chocolate. Return to microwave and heat at 50 percent power until melted and smooth, about 1 minute longer. Let cool slightly, about 5 minutes.
Add softened butter, sugar, cocoa powder, and salt to food processor, and lock lid into place. Turn on processor and process until smooth, about 30 seconds. Stop processor.
Remove lid and use rubber spatula to scrape down sides. Add cooled chocolate and vanilla and lock lid back into place. Turn on processor and process until smooth, 10 to 15 seconds. Stop processor.