Preheat oven to 180°C and pulse hazelnuts in a food processor until coarsely ground.
Bake ground nuts for 10-12 minutes until golden-brown, then cool and stir in 100g caster sugar and 25g cornflour.
Reduce oven temperature to 150°C.
Draw three 21cm circles on parchment-lined trays and position racks in upper, middle, and lower thirds.
Whisk egg whites with salt and remaining 200g sugar until stiff and glossy, then fold in hazelnut mixture.
Pipe meringue spirals onto the templates using a 1.5cm plain nozzle.
Bake for 1 hour, then cool in the oven with the door open for 45 minutes.
Make ganache by pouring simmered cream over chopped chocolate, stirring until smooth, and chilling until thick.
Make custard by whisking egg yolks, sugar, coffee essence, and cornflour into boiling milk until very thick.
Whisk whipping cream to soft peaks and fold in the cooled coffee custard.
Create praline by melting sugar to 300°C, adding nuts and lemon juice, and forming 12 clusters.
Layer meringue with custard, coat the sides with custard and chopped nuts, and pipe ganache swirls on top.