This impressive centrepiece is three nutty meringue layers sandwiched with a dark chocolate ganache and rich creamy coffee mousse.
Make the hazelnut meringue. Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Tip the hazelnuts into a baking dish and toast in the oven for about 5–8 minutes, until golden. Leave until cold, then transfer to the bowl of a food processor.
Add the icing sugar, half the caster sugar, and the cornflour. Pulse to a coarse sand texture (don’t over chop).
Reduce the oven to 120°C/100°C fan/235°F/Gas ½–1. Put the egg whites and salt in the bowl of a stand mixer fitted with the whisk and whisk to soft peaks. Sprinkle over the remaining caster sugar, then whisk for a further 30 seconds to stiff peaks.
Sprinkle over one third of the hazelnut mixture and fold in lightly. Repeat twice more. Add a spoonful of the hazelnut/meringue mixture to the cooled melted butter and mix well, then gently fold into the remaining hazelnut/meringue mixture.
Transfer to a large piping bag fitted with a 1.5cm plain nozzle. Line the baking sheets with the paper drawn with 20cm circles. Pipe 3 spiral discs inside the circles, starting at the centre and spiralling outwards. Fill any gaps.
Bake the discs for about 1½ hours, until very light golden, rotating the sheets after 50 minutes. Remove from the oven and set the sheets on a wire rack. Leave until cold, then peel the discs off the paper.
Make the ganache. Heat the cream until almost boiling, then pour it over the chopped chocolate and leave for 2 minutes to melt. Stir until very smooth, then stir in the butter. Leave to cool until very thick, but fluid.
Spoon half the cooled ganache into the centre of each 20.5cm circle on the baking paper sheets and gently spread out to create 2 even, flat discs. Chill for about 30 minutes, until firm enough to peel away.
Make the coffee mousse. Soak the gelatine in cold water for about 10 minutes, until soft.
Put the egg yolks, sugar and cornflour into a heatproof bowl and beat until very smooth, thick and creamy.
Heat the milk with the coffee in a medium pan over a medium heat until steaming hot, then pour onto the yolk mixture in a steady stream, using a hand whisk to whisk continuously until combined.
Pour the mixture back into the pan and whisk continuously until the mixture boils and thickens. Remove the pan from the heat.
Squeeze out the gelatine, then whisk it into the hot coffee mixture until completely smooth. Leave to cool, stirring frequently.
When the mixture is cold but still fluid, whisk the cream to stiff peaks, then gently fold into the coffee mixture.
Line the sides of the springform tin with the acetate and secure with sticky tape, then assemble the dessert. Place one meringue disc on the base of the tin and very gently place a disc of ganache on top.
Spoon half the coffee mousse onto the centre of the ganache disc and gently ease it to the edges to make an even layer about 2.5cm deep. Freeze for about 15 minutes, until set. Stir the remaining coffee mousse from time to time to keep it fluid.
Remove the tin from the freezer and repeat the layers. Return to the freezer for 15 minutes, then top with the last meringue disc. Freeze for 2–3 hours, until very firm.
Make the Chantilly cream. Whip the cream with the sifted icing sugar until it stands in stiff peaks.
To finish the dessert, carefully unclip the tin and remove the acetate, then set the dessert on a serving platter. Cover the top and sides with a thin layer of the whipped cream.
Transfer the remainder to a large piping bag fitted with a star nozzle. Pipe a small rope of cream around the top edge, and another looser one around the base. Decorate with grated chocolate. Chill for at least 1 hour, preferably overnight, before serving.