1Heat oven to 350°F (325°F for dark or nonstick pans). Grease or spray bottoms and sides of two 9- or 8-inch round cake pans with baking spray with flour.
2Make, bake and cool cake as directed on box for 8- or 9-inch rounds. Refrigerate layers about 45 minutes for easier handling.
3Meanwhile, in medium microwavable bowl, microwave chocolate chips and whipping cream uncovered on High 1 minute 30 seconds to 2 minutes 30 seconds, stirring every 30 seconds, until cream is hot. Stir until chocolate is melted and smooth. Refrigerate 30 to 40 minutes or until cool. Stir just until mixture is thick enough to spread but is still glossy.
4Slice each cake layer in half horizontally to make a total of 4 layers. Reserve 1 top layer. Place 1 cake layer bottom, cut side up, on serving plate. Spread with thin layer of frosting; sprinkle with 1/3 cup of the toffee bits. Repeat with 2 more layers. Place reserved layer on top, rounded side up. Frost side and top of cake with remaining frosting. Sprinkle remaining toffee bits on top of cake. Garnish with pirouette cookies. Store covered in refrigerator.