Make the meringue the day before: preheat the oven to its coolest setting.
Line a large baking tray with non-stick baking paper.
Mix the icing sugar, hazelnuts and ground almonds together in a bowl.
In another bowl whisk the egg whites until firm.
Using a metal spoon, gently fold in the icing sugar mixture to the egg whites, being careful not to over mix.
Spoon the mixture into a piping bag. Pipe the mixture onto the prepared baking tray into a round a little smaller than the mousse ring.
Place the tray in the oven for at least 4 hours, or until completely dry and crunchy.
Make the coffee mousse on the day of serving: heat 375g sugar in a heavy-based pan over a low heat, without stirring until caramelised and pale gold.
Add the hazelnuts, tilting the pan to coat them well with the caramel. Transfer to a greased tray and leave until cold and set.
. When cold, break into pieces and process in a food processor until finely ground and almost powdery.
. Whisk the egg whites until firm and whisk in 50g sugar.
. Add the warm coffee to the warm cream (make sure the cream is warm not hot) and mix gently into the soft butter. Stir in the caramelised hazelnut mixture. Gently fold in the whisked egg whites, stirring gently until well mixed. Spoon into a piping bag.
. Place the meringue disc in the bottom of a 22cm diameter mousse or flan ring. Pipe in the coffee mousse to come halfway up the sides of the mousse ring.
. To make the chocolate mousse: heat the sugar in a pan with water until the sugar is completely dissolved. Bring to the boil and cook for 2-3 mins.
. Whisk the egg yolks with an electric whisk at high speed. Pour the hot sugar syrup on the yolks, whisking at medium speed and then increase to high speed and whisk until the mixture is cold and fluffy.
. Meanwhile, melt the chocolate in a bain-marie or heatproof bowl over a pan of simmering (not boiling) water. Remove from the heat.
. Warm the cream and add to the melted chocolate.
. Add the egg yolk mixture to the chocolate mixture, stirring until combined. Gently fold in the whipped cream.
. Fill the rest of the mousse ring with the mousse and level it with a palette knife. Leave in the fridge to set for at least 4 hours 20. To make the chocolate glaze: melt the dark chocolate as before. Heat the cream and gently stir in the glucose syrup. Using a wooden spoon, stir into the melted chocolate.
. Using a blow torch, gently warm the outside of the mousse ring for a few seconds. This makes it easier to remove the cake. Release the cake from the ring and place on a wire rack.
. Cover the cake with warm chocolate glaze and smooth the surface with a palette knife. Leave to set for 20 minutes before transferring to a serving plate.
. Decorate the cake with chocolate pieces and serve.