For the chocolate hazelnut cake:

1. Pre-heat the oven to 350 F / 180 C.
2. Grease and line three 7-inch / 18-centimeter round cake pans and set aside.
3. Add the hazelnut meal, sugar, flour, cocoa, baking powder, soda and salt to a large mixing bowl. Whisk together until the dry ingredients are evenly combined.
4. In a separate large mixing bowl, whisk together the eggs, milk, melted butter, kahlua and vanilla extract.
5. Add the wet ingredients to the dry ingredients and stir with a wooden spoon to just combine then pour in the boiling water. Mix well until combined.
6. Divide the cake batter evenly between the prepared cake pans and bake until a skewer inserted into the center comes out clean, roughly 30 to 35 minutes.
7. Let the cakes cool in their pans for 10 minutes before inverting them onto a wire rack to cool completely.
8. The cakes can be made one day in advance. Keep tightly wrapped at room temperature.

For the Gianduja Hazelnut Crunch:

1. Line a baking sheet with parchment paper and set aside.
2. Add the Gianduja and butter to a medium heatproof bowl. Set the bowl over a saucepan of barely simmering water, making sure that the water does not touch the base of the bowl. Heat the mixture, stirring often, until it is melted and smooth then remove from the heat.
3. Stir in the Nutella and chopped hazelnuts followed by the rice puffs.
4. Spread the mixture out onto the prepared baking sheet and freeze until set, about 30 minutes.
5. Once set, use your hands to break the crunch into small pieces.
6. The crunch can be made one day in advance and kept covered in the fridge or freezer.

For the Gianduja Chocolate Frosting:

1. Combine both the chocolates and corn syrup in a medium bowl. Bring the cream to a light boil in a small saucepan then pour it over the chocolate bowl. Let it sit to slightly melt for 2 minutes before stirring until smooth.
2. Once the mixture is smooth transfer it to the bowl of a stand mixer fitted with the beater attachment, or with hand-held electric beaters, beat until cool, 6 to 8 minutes.
3. Add the butter a few pieces at a time, beating well to incorporate between additions. Continue to beat until the mixture is thick, smooth and very light in colour, about 8 to 10 minutes with a stand mixer, and up to 15 minutes with a hand-held mixer. If the frosting seems too thin, stop occasionally and chill to firm the butter solids before continuing to beat.

For Assembly:

1. Level out the cooled cake layers. Fix the first layer on a large stand or platter and spread about 1 cup of the frosting over it then sprinkle with ½ scant cup of the Hazelnut Gianduja crunch. Place the second cake layer on top; press gently to adhere and repeat the process with another 1 cup of the frosting and ½ scant cup of the crunch. Top with the third cake layer.
2. Spread a thin layer of the frosting over the top and sides of the cake to crumb coat and chill the cake for about 30 minutes or until set. The frosting bowl can also be kept in the refrigerator whilst you wait for the crumb coat to harden.
3. Spread the remaining frosting over the top and sides of the cake then decorate the top of the cake with more of the Hazelnut Gianduja crunch. Chill the cake until completely set, 2 to 3 hours before serving.
4. You can keep the cake chilled and covered for up to three days. Let it stand at room temperature for 30 minutes before serving.