This glorious gateau combines layers of nutty meringue, praline buttercream and chocolate ganache. It’s naturally gluten-free, too.
Pre-heat the oven to 180°C/160°C fan/350°F/Gas 4 and line two 30cm x 20cm Swiss roll tins with baking parchment.
Tip the almonds and hazelnuts into the bowl of food processor and pulse until finely ground. Spread the nuts in a layer in a roasting tin and cook for 10–12 mins, stirring every 3 minutes until golden. Remove from the oven, transfer to a large bowl and leave to cool. When cool, stir in 100g of the caster sugar and the cornflour.
Reduce the oven temperature to 150°C/130°C fan/300°F/Gas 2. Pour the egg whites into the bowl of an electric mixer with the whisk attachment and whisk on medium speed for about two minutes, until white and frothy. Increase the speed and add the remaining 200g caster sugar, 1 tablespoon at a time, to make a stiff, glossy meringue. Gently fold the roasted nut mixture through the meringue.
Divide the meringue between the two Swiss roll trays and spread out with a palette knife. Bake for 45 minutes–1 hour, until lightly golden and firm to touch. Turn off the oven and open the oven door so it is just ajar, then leave the meringues inside until completely cool.
Remove from the oven and carefully invert onto a sheet of baking parchment. Carefully remove the baking parchment form the bottom of the meringues and leave to cool completely.
For the chocolate ganache, break the chocolate into pieces and place in a heatproof bowl. Pour the cream into a pan and heat until simmering. Remove from the heat and pour over the chocolate. Stir until the mixture is smooth. Leave to cool and then chill, until thick enough to spread and pipe.
For the praline, toast the blanched almonds in a dry frying pan until golden. Boil the caster sugar and water together in a pan until it turns golden or reaches 170°C/338°F on a sugar thermometer. Remove from the heat and add the toasted almonds, stir well and tip onto a baking sheet lined with silicon paper. Leave to cool, then tip into the bowl of a food processor and blitz to a fine powder.
For the French buttercream, pour the water into a heavy-based pan with the sugar and cream of tartar. Bring to the boil and boil rapidly until it reaches 115°C/239°F on the sugar thermometer. Pour the egg yolks into the bowl of an electric mixer with the whisk attachment. With the mixer running, gradually add the sugar syrup and continue whisking for 5–10 minutes until the mixture is thick and cool. Gradually add the butter bit by bit, whisking continuously.
Fold in the praline powder and chill until needed.
To assemble, toast the flaked almonds in a large dry frying pan until golden brown. Carefully cut the meringue sheets in half lengthways so you have 4 sheets of meringue each measuring 30cm x 10cm. Place one sheet of meringue on a serving plate and spread with one quarter of the praline buttercream. Top with a second sheet of meringue and spread with one third of the chocolate ganache. Top with a third sheet of meringue and spread with another quarter of the praline buttercream. Top with final sheet of meringue.
Spread the remaining praline buttercream over the top and sides and press the toasted flaked almonds onto the sides of the gateau. Spoon the remaining ganache into a piping bag fitted with a small (No.33) star nozzle. Pipe the chocolate ganache around the top edge of the cake and in 7 diagonal lines across the top of the cake. Fill the gaps between the diagonal piped lines with alternating roasted chopped hazelnuts and slivered pistachios.