In large bowl, beat mascarpone cheese and 1/2 cup powdered sugar with electric mixer on low speed until mixed. Beat on medium speed about 1 minute or until smooth. Gradually beat in whipping cream, beating on medium speed 1 to 2 minutes or until stiff peaks form. Beat in 3 tablespoons coffee liqueur. Spread mixture evenly over top of cooled cake. Chill cake 1 hour. Sprinkle with cocoa just before serving.