2) Prepare the cake mix according to package instructions, divide between the 2 cake tins and bake according to the package instructions. Remove from the oven and leave to cool on a wire rack.
3) Place the toasted nuts close together in a single layer on a greaseproof paper-lined baking sheet. Combine the sugar and water in a small saucepan over a medium-high heat, stirring until the sugar has dissolved.
4) Bring to the boil and cook until the sugar is light brown, about 8 minutes. Let the bubbles subside then pour the caramelized sugar over the nuts. Place the baking sheet in the refrigerator and let the sugar nut mixture cool until hard, about 30 minutes.
5) When the sugar nut mixture is hard and cool, top with another piece of parchment paper and pound into small pieces, or place the sugar nut mixture on a cutting board and cut into small pieces. Set aside.
6) Put the mascarpone cheese, cream, powdered sugar and vanilla into a large mixing bowl and whip into soft peaks using an electric mixer. Fold the sugar nut mixture into the whipped cream.
7) Place the chocolate, sugar and zest in a food processor. Process the mixture until the chocolate is finely ground.
8) Put one cake layer on a serving plate. Top with 2cm of the whipped cream crunch mixture and add the second layer of cake. Continue frosting the entire cake with the remaining whipped cream crunch mixture. Sprinkle the top and sides of the cake with the ground chocolate. Serve.