Melt the chocolate in a double boiler or in the microwave. Remove from heat and let cool.
Combine cocoa and the boiling water, stirring until cocoa has dissolved. Set aside and let cool.
With an electric mixer on medium-high speed, beat softened butter, confectioners' sugar, and salt until pale and fluffy. Reduce speed to low. Gradually add melted and cooled chocolate, beating until combined and scraping down sides of bowl as needed.
Beat in vanilla (if using)
Beat in the cocoa mixture. If not using immediately, frosting can be refrigerated up to 5 days, or frozen up to 1 month, in an airtight container. Before using, bring to room temperature, and beat on low speed until smooth again.