Hazelnut-chocolate cake

1.For mocha meringue, preheat oven to 100C, and line 2 oven trays with baking paper. Whisk eggwhite, caster sugar and ½ tsp sea salt in an electric mixer until glossy soft peaks form (3-4 minutes). Sieve in icing sugar, cocoa and cornflour, fold to combine, then fold in coffee. Spread mixture 1cm thick on prepared trays, then bake until crisp but not coloured (1¼-1½ hours). Cool completely on trays, then store between layers of baking paper in an airtight container until required. Meringue can be made 2 days ahead; crumble it just before serving.

2. For truffle filling, bring cream and half the coffee liqueur to the boil. Remove from heat, add chocolates and stand for 5 minutes, then whisk until smooth. Whisk yolks and remaining liqueur in a bowl over a saucepan of simmering water until pale and fluffy (2-3 minutes), then whisk in chocolate mixture, transfer to an electric mixer and whisk until cooled to room temperature (5-6 minutes). Transfer to a container, cover and refrigerate overnight to set. Filling can be made a week ahead.

3.Preheat oven to 180C, and butter a 10cm diameter cake tin, a 16cm-diameter cake tin and a 21cm-diameter cake tin, line the bases with baking paper and dust with flour. Whisk eggs, egg yolks, sugar and a good pinch of salt in an electric mixer until thick, pale and tripled in volume (5-6 minutes). Meanwhile, triple-sieve flour, cornflour and baking powder into a bowl, then stir in hazelnut meal. Transfer egg mixture to a large bowl, fold in hazelnut mixture in 3 batches, then fold in butter and oil. Pour into prepared cake tins, filling to 1.5cm below rims and bake, swapping tins occasionally, until centres spring back when lightly pressed (40-45 minutes for small cake; 50-55 minutes for medium cake; 50 minutes to 1 hour for large cake). Cool in tins for 5 minutes, then turn out onto wire racks to cool completely (1 hour).

4.For chocolate ganache, bring cream just to the boil in a saucepan, remove from heat, add chocolate, butter and glucose, stand for 5 minutes, then whisk until smooth and just combined (don’t overwhisk or mixture can split). Transfer to a container and refrigerate, stirring occasionally, until spreadable (5-6 hours).

5.Whisk mascarpone and brown sugar in a bowl to firm peaks (1-2 minutes; don’t overwhisk or it will curdle) and refrigerate until required.

6.Halve each cake horizontally and brush cut sides with coffee mixture. Spread or pipe truffle filling to 1cm thick on each base, top with crushed mocha meringue, spread or pipe mascarpone over to 1cm thick and sandwich with top halves. Spread tops and sides of cakes with ganache with an offset palette knife dipped in hot water. Stack cakes, ascending to smallest, decorate with chocolate curls and gold cachous, and serve.