Set oven racks in the upper, middle, and lower positions. Heat the oven to 350°F or 325°F convection. Line 3 baking sheets with parchment and trace an 8-inch circle on each one.

Pulse the hazelnuts in a food processor until coarsely ground. Then spread them in a roasting pan and toast for 10 to 12 minutes until golden, stirring every 3 minutes. Let cool completely.

Stir the sugar and cornstarch into the cooled nuts. Whisk the egg whites with the salt on medium speed until frothy. Gradually add the remaining sugar until stiff and glossy. Fold in the nut mixture gently.

Pipe the meringue in a spiral to fill each traced circle. Bake for 1 hour, swapping the top and bottom sheets halfway through, then let cool inside the oven.

For the ganache, place chocolate in a bowl, heat cream to a simmer, pour over chocolate, and stir until smooth. Chill until firm enough to pipe.

For the coffee custard, bring milk to a boil. Whisk yolks, sugar, and coffee essence together, then whisk in cornstarch. Gradually add the hot milk. Return to the pan, boil, and simmer until thick. Cool for at least 1 hour.

For the praline, melt sugar until deep golden in a small pan. Stir in whole hazelnuts and lemon juice, then pour onto parchment to set.

Whip the chilled cream to soft peaks and fold half into the cold custard, then fold in the rest. Refrigerate the filling.

Assemble the cake by placing one meringue disc on a plate, spreading one third of the coffee cream, and layering with the second disc and half of the remaining cream. Top with the third disc.

Coat the sides with the rest of the cream, press chopped toasted hazelnuts onto the sides, and pipe ganache swirls on top. Add praline clusters on top as garnish. Chill for about 2 hours before serving.