Preheat the oven to 180°C / gas mark 4. Butter and flour two (20cm) cake tins. Prepare the cake mix according to package instructions. Divide the batter between the two cake pans and bake according to package instructions. Remove from the oven and let cool on a wire rack.
For the icing and garnish:
1) Place the toasted nuts close together in a single layer on a parchment-lined baking sheet. Combine the sugar and water in a small saucepan over medium-high heat. Stir the sugar mixture until dissolved. Bring to a boil and let cook until the sugar is light brown, about eight 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. 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.
2) Put the mascarpone cheese, cream, icing sugar, and vanilla essence into a large mixing bowl. Using an electric mixer whip the cream mixture to soft peaks. Fold the crunch mixture into the whipped cream.
3) Place the chocolate, sugar and zest in a food processor. Process the mixture until the chocolate is finely ground. Set aside.
Put one cake on a serving plate. Top with 2 1/2 cm of the whipped cream crunch mixture. Top with the second layer of cake and 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.