For the sponge cakes, preheat the oven to 180C/350F/Gas 4 and lightly grease and line the base of two 25cm/10in loose bottomed cake tins.
For the hazelnut sponge, in a clean bowl, whisk the egg whites and 50g/1¾oz of the icing sugar until stiff peaks form when the whisk is removed.
In a separate bowl, whisk together the remaining icing sugar and whole eggs until thickened and pale in colour.
Fold in the ground hazelnuts gently, sift in the flour and mix in the melted butter.
Finally, gently fold in the whisked egg whites until incorporated.
Spoon the mixture into one of the prepared cake tins, then tap to remove any air pockets.
Place in the oven for 16 minutes, or until risen and just firm to the touch.
Remove from the oven and cool briefly before tipping out onto a rack to cool completely.
For the chocolate sponge, place the eggs and sugar in a bowl and whisk for 4-5 minutes, or until thickened and very pale.
Sift in the flour and cocoa powder and add the melted butter. Gently fold the batter to combine.
Spoon the mixture into the remaining cake tin, then tap to remove any air pockets.
Bake in the oven for 16 minutes, or until risen and just firm to the touch – do not overcook this sponge, as it will become dry.
Remove from the oven and cool briefly before tipping out onto a rack to cool completely.
For the hazelnut cream, in a bowl combine the cream and hazelnut paste. Whisk until soft peaks form when the whisk is removed. Place in the fridge until ready to use.
For the coffee flavoured meringue buttercream, place the sugar and 125ml/4½fl oz water in a pan and slowly bring to the boil. Continue to heat until the syrup reaches 110C/230F on a sugar thermometer.
Meanwhile, place the egg whites in the bowl of a food mixer (make sure it’s scrupulously clean) and add a squeeze of lemon juice. Whisk until firm peaks form.
Keep checking the temperature of the syrup and as soon as it reaches 121C/250F, remove the pan from the heat and immediately resume whisking the egg whites.
With the beater at its lowest speed, pour in the sugar syrup, in a slow steady stream, making sure it is clear of the whisk (the syrup is extremely hot and could splash out of the bowl if it hits the whisk).
Continue beating for about five minutes, until the meringue is tepid.
Add the butter and continue to beat until it is all incorporated. Beat in the cold coffee and set aside to cool completely.
To assemble the gateâu, horizontally cut each sponge cake into three layers to form six thin slices.
Place one sponge layer on a serving plate and drizzle over a spoonful of cognac. Spread over a thin layer of hazelnut cream.
Top with another layer of sponge, cognac and cream and repeat with the remaining sponges until you have a stack of six sponges filled with five layers of cream.
To decorate, using a pallet knife spread the coffee flavoured meringue buttercream around the sides of the cake and over the top, finishing with a swirl pattern.
Use a little buttercream to stick the marshmallows to the meringue shells. Dust with cocoa powder, then place on the top of the cake in a little cluster.
Slice and serve.