How to Make It
Preheat the oven to 250° and line 2 large baking sheets with parchment paper. Using the bottom of a 9-inch springform pan, draw a circle in the center of each sheet of parchment paper.
In a small bowl, combine the coffee extract and water. In another small bowl, whisk the hazelnut meal with the cornstarch and cinnamon. In the bowl of an electric stand mixer fitted with the whisk attachment, beat the egg whites at medium-high speed until soft peaks form. Gradually add the confectioners’ sugar and beat at high speed until the whites are stiff, 2 to 3 minutes. Gently fold in the hazelnut mixture, then the coffee extract.
Scrape half of the meringue into the center of each traced circle; using an offset spatula, spread in an even layer, leaving a 1/2 -inch border inside the circle. Bake the meringues in the lower and upper thirds of the oven for about 1 hour, until lightly browned and crisp; shift the sheets halfway through baking. Transfer to racks to cool.
In a heatproof large bowl set over a medium saucepan of simmering water, heat 1/2 cup of the cream with the coffee extract until hot. In a medium bowl, whisk the egg yolks. Very gradually whisk the hot cream mixture into the yolks, then add the mixture to the large bowl and cook over the simmering water, stirring constantly, until the custard is hot and slightly thickened, about 7 minutes. Fold in both chocolates and a generous pinch of salt and let stand over the simmering water until the chocolate is nearly melted, about 3 minutes. Remove from the heat and stir gently until smooth. Let cool.
In a large bowl, using an electric mixer, beat the remaining 1 1/2 cups of cream until stiff peaks form. Using a spatula, very gently fold the whipped cream into the cooled chocolate custard until no streaks remain. Refrigerate until just chilled, 30 minutes.
Trim the meringues to fit into a 9-inch springform pan. Set 1 meringue in the pan. Spread the mousse over the meringue in an even layer and top with the remaining meringue. Refrigerate the cake for at least 6 hours or overnight. Unmold the cake and dust with cocoa powder or confectioners’ sugar. Cut the cake into wedges with a hot knife and serve.