Preheat the oven to 200C/180C Fan/Gas 6. Grease and line three 18cm/7in sandwich tins with baking paper.
For the cake, sieve the flour, sugar, cocoa powder and baking powder into a large mixing bowl. Add the eggs and margarine and beat on high speed for 10 seconds using an electric mixer. Scrape down the sides to make sure everything is incorporated. Add the milk and mix for another 10 seconds on high until well combined.
Divide the mixture evenly between the three sandwich tins and bake on the middle shelf of the oven for 15-20 minutes or until springy to the touch and starting to come away from the sides of the tins. Remove from the tins and place on a wire rack to cool.
For the icing, add the sugar and evaporated milk to a large saucepan set over a medium heat. Stir until it starts to simmer, then stop stirring and allow to bubble for 6 minutes. Remove from the heat and set aside to cool slightly. Add the chocolate and butter to the warm mixture and stir until melted and incorporated. Cool until it reaches a spreadable consistency. Transfer the icing to a large piping bag fitted with a large closed star nozzle.
To decorate, stack the three layers of cake with 3–4 piped rings of icing in between each layer. Cover the top with small swirls of icing then add the raspberries and sprinkle the roasted chopped hazelnuts over the cake.
In two separate bowls, melt three-quarters of the white chocolate and dark chocolate. Once melted and cooled slightly, add the remaining quarter of each chocolate and stir until combined. When the melted chocolate becomes tacky, spread the dark chocolate in a thin layer on a large piece of non-stick paper, acetate or a silicone mat. Drizzle lines of the white chocolate across the dark. Leave to set completely then cut into shards. Use the shards to decorate the top of the cake.