Dark Chocolate Buttercream
1/2 cup plus 1 tablespoon unsweetened Dutch-process cocoa powder
2 1/4 cups (4 1/2 sticks) unsalted butter - room temperature
1/2 cup plus 1 tablespoon boiling water
1/4 cup confectioners’ sugar - sifted
1/4 teaspoon salt
1 1/2 pounds semisweet chocolate - melted and cooled
Combine cocoa and the boiling water, stirring until cocoa has dissolved. Beat butter, confectioners’ sugar, and salt until pale and fluffy. Reduce speed to low. Add melted and cooled chocolate, beating until combined. Scrape down sides of bowl if needed. Beat in the cocoa mixture. Frosting can be refrigerated up to 7 days, or frozen up to 1 month (before using, bring to room temperature, and beat on low speed until smooth again).