A truly impressive, gluten-free cake, this is fit for the most glamorous celebration: nutty meringue layers, sandwiched with a rich and creamy coffee crème patissiere and decorated with swirls of ganache and hazelnut praline. You can make the dacquoise in advance – and it will still be delicious the next day.
Arrange your oven racks in the upper, middle and lower thirds of the oven (you can use the grill tray on the bottom of the oven if you don't have a third rack). Heat the oven to 180°C/160°C fan/350°F/Gas 4. Using a 21cm dinner plate or cake tin as a guide, draw a circle on the paper lining each of the baking sheets. Set aside for now.
To make the dacquoise, put the hazelnuts into the bowl of a food processor and 'pulse' just until coarsely ground. Spread the nuts in an even layer in a roasting tin and toast in the oven for 10–12 minutes until golden, stirring every 3 minutes. Remove the tin from the oven. Tip the nuts into a large heatproof bowl and leave to cool. Reduce the oven temperature to 150°C/130°C fan/300°F/Gas 2.
When the nuts are cold, stir in 100g of the caster sugar and the cornflour. Pour the egg whites into the bowl of an electric mixer fitted with the whisk attachment. Add the salt. Whisk at medium speed for about 2 minutes until the whites are frothy. Increase the speed and continue to whisk while adding the remaining 200g sugar, a tablespoon at a time, to make a stiff glossy meringue that stands in peaks when the whisk is lifted out. Gently fold the toasted hazelnut mixture through the meringue using a large metal spoon or plastic spatula.
Transfer the meringue to a piping bag fitted with a 1.5cm plain tube. Starting in the centre of each drawn circle, pipe a spiral of meringue onto each prepared baking sheet. Place in the heated oven and bake for 1 hour, swapping the top and bottom baking sheets halfway through so the meringue discs cook evenly. Turn off the oven and open the oven door so it is just ajar, then leave the discs inside until completely cool.
Meanwhile, make the chocolate ganache. Break up the chocolate into even-sized pieces and put into a heatproof bowl. Pour the cream into a pan and heat until simmering. Pour the hot cream over the chocolate and stir gently until smooth. Leave to cool, then cover the bowl and chill until really thick and with a piping consistency (like whipped cream). Spoon the ganache into the clean and dry piping bag fitted with a star tube. Set aside at room temperature until needed.
For the coffee filling, pour the milk into a heavy-based pan and slowly bring to the boil over a low heat. Meanwhile, put the yolks, sugar and coffee and chicory essence into a medium-sized heatproof bowl and whisk to combine. Whisk in the cornflour a tablespoon at a time to make a smooth, thick paste. Pour on the hot milk in a thin steady stream, whisking constantly. Pour the mixture into the pan and whisk until it comes to the boil, then lower the heat so the mixture just simmers. Whisk for 2–3 minutes until thickened and very smooth. Remove the pan from the heat and leave to cool for at least an hour.
Whip the cream until it stands in soft peaks. Whisk half of the whipped cream into the cold coffee mixture, then gently fold in the rest. Cover and chill until needed.
To make the praline, put the caster sugar in a small frying pan and melt, without stirring. Continue cooking until it turns to a golden caramel or it reaches 300°C/570°F on the sugar thermometer. Add the whole hazelnuts and lemon juice and stir well with a wooden spoon, then tip onto a baking sheet lined with baking paper or silicone paper. Working quickly before the caramel sets, use 2 teaspoons (don't touch the caramel as it will be very hot!) to form 12 equal clusters of hazelnut praline for the decoration. Leave to cool and set.
To assemble the dacquoise, set one of the meringue layers on a large, flat serving plate and spread with a third of the coffee cream. Place another meringue layer on top and spread with half the remaining coffee cream. Top with the last meringue layer. Spread the remaining coffee cream over the sides of the cake, leaving the top bare. Press the chopped toasted nuts onto the sides of the dacquoise. Pipe 12 swirls of chocolate ganache on the top of the cake and decorate with the praline clusters.