Chocolate Buttercream icing

Recipe from epicurious.com
Frosting:

• 10 ounces bittersweet chocolate (do not exceed 61% cacao), chopped
• 1 1/2 cups (3 sticks) unsalted butter, room temperature
• 3 cups powdered sugar
• 1 tablespoon vanilla extract

Place chopped chocolate in medium metal bowl; set bowl over saucepan of simmering water and stir until chocolate is melted and smooth. Carefully remove bowl from over water; let melted chocolate cool until lukewarm, stirring occasionally.

Using electric mixer, beat butter in large bowl until smooth and creamy. Sift powdered sugar over butter and beat until well blended, about 2 minutes. Beat in vanilla. Add melted chocolate and beat until well blended and smooth, occasionally scraping down sides of bowl.

Place 1 cake layer on platter. Spread 3/4 cup frosting over top of cake layer to edges. Top with second cake layer; spread 3/4 cup frosting over. Top with third cake layer. Spread remaining frosting decoratively over top and sides of cake.