Instructions:
Step 1: Preheat oven to 350°F (175°C). Prepare chocolate cake batter according to box instructions and divide evenly into two or three 8-inch round cake pans. Bake as directed and cool completely.
Step 2: To make the hazelnut mousse, beat heavy cream and powdered sugar until soft peaks form. Gently fold in Nutella until fully incorporated. Chill until ready to use.
Step 3: For the frosting, beat softened butter until creamy. Add powdered sugar, cocoa powder, milk, and vanilla extract. Beat until smooth and fluffy.
Step 4: Once cake layers are cool, slice each in half horizontally if needed to create more layers.
Step 5: Layer the cake by spreading hazelnut mousse between the cake layers, adding a thin layer of crushed cookies between mousse layers if desired for crunch.
Step 6: Frost the outside of the cake with the chocolate frosting.
Step 7: Top with chopped hazelnuts and refrigerate for at least 1 hour before serving.
Storage and Reheating Tips:
Store the cake in the refrigerator, covered, for up to 4 days. Bring to room temperature for 20–30 minutes before serving for the best texture. Do not microwave. This cake also freezes well without the hazelnut topping—wrap tightly and freeze for up to 2 weeks.
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