Preheat the oven to 180℃ /160℃ fan/gas mark 4. Lightly grease a 20cm deep springform cake tin and a 20cm sandwich tin.
To make the chocolate cake.
Beat the butter and sugar together until light and creamy. Add the eggs one at a time, beating well after each addition, add the cocoa powder with the last egg. Sift in the flour and fold in, then fold in the melted chocolate.
Pour into the spring form tin, level top, then make a slight dip in the centre. Bake for 50 minutes – 1 hour or until springy to the touch.
Make the hazelnut cake.
Beat the butter and sugar together until light and creamy. Add the eggs one at a time, beating well after each addition. Sift in the flour and fold in, along with the chopped hazelnuts.
Pour into the sandwich tin and spread level. Bake for 25–30 minutes or until springy to the touch.
When the cakes are cooked allow to cool in the tins for a few minutes, then transfer to a wire rack to cool completely. Split the chocolate cake into two layers.
To make the ganache.
Break the chocolate into small pieces and place in a bowl. Heat the cream in a saucepan until almost boiling, then pour onto the chocolate, stirring. Continue to stir until all the chocolate has melted and combined with the cream.
Whisk until the ganache is cool and thick enough to pipe.
To assemble the cake.
Use about three-quarters of the ganache to sandwich the two chocolate cakes together with the hazelnut cake in between. Spread ganache over the side of the cake and transfer the cake to a baking sheet or tray.
Spoon the remaining ganache into a piping bag fitted with a star nozzle and pipe rosettes to completely cover the top of the cake.
Cut the transfer sheets into strips and long enough to go around the cakes. Temper the chocolate by melting about ⅔ in a bowl slowly over a pan of gently simmering water. remove from the heat and stir in the remaining chocolate. Stir until melted. (Follow the link in post for more details on how to temper chocolate.)
Spread the chocolate over the transfer sheets going right up to the edges, then carefully lift up and place around the edge of the cake. Allow the chocolate to set.
Carefully remove the transfer sheets and carefully transfer the cake to a serving plate.