Frosting-Place the butter, cocoa powder, and chocolate syrup in a large mixing bowl. Blend with an electric mixer on low speed until just combined, 30 seconds. Stop the mixer and add confectioners sugar, milk, and vanilla. Blend with the mixer on low speed until the sugar is incorporated, 1 minute. Increase the speed to medium and beat until fluffy, 1 minute more. Frost your layers and sides and serve.