3sticksunsalted butter1 1/2 cups, room temperature
Place chocolate in a heatproof bowl set over (but not touching) simmering water. Turn off heat; stir occasionally until chocolate has melted completely, about 15 minutes. Set bowl on countertop, and let chocolate cool to room temperature, 25 to 30 minutes.
Meanwhile, combine cocoa powder and boiling water in a small bowl; stir until cocoa is dissolved.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter, confectioners' sugar, and salt on medium-high speed until light and fluffy, 3 to 4 minutes.
Add melted chocolate; beat on low speed until combined, 1 to 2 minutes, scraping down the sides of the bowl as needed.
Beat in cocoa mixture. Use immediately or chill until ready to use. Bring frosting back to room temperature and beat until smooth before using.