Heat oven to 180°C (170°C for dark or nonstick pans). Make, bake and cool cake as directed on box using two 20cm round cake pans.
Place 1 cake layer, rounded side down, on serving plate. Spread with about 75 grams frosting. Top with second layer, rounded side up. Frost side and top of cake with remaining frosting.
In a 1-liter saucepan, heat whipping cream over medium heat until hot (do not boil); remove from heat. Stir in chocolate chips until melted and smooth. Let stand 5 minutes. Carefully pour chocolate mixture onto top center of cake; spread to edge, allowing some to drizzle down side. Garnish top of cake with toffee candy. Refrigerate about 1 hour or until chocolate is set. Store covered in refrigerator.