In a small bowl, sprinkle the gelatin over the water and let stand until softened. In a medium saucepan, bring the milk and cream to a simmer. In a medium heatproof bowl, whisk the egg yolks with the sugar. Gradually whisk the hot liquid into the egg yolks, then pour the mixture into the saucepan and whisk over low heat until thickened, about 4 minutes. Remove from the heat and whisk in the softened gelatin. Stir in both chocolates until melted. Transfer the cream to a bowl, press plastic wrap directly onto the surface and refrigerate until chilled, about 2 hours.
Preheat the oven to 350°. In the bowl of a standing electric mixer fitted with the whisk, beat the egg yolks with 1/4 cup of the sugar at medium-high speed until thick and pale, about 3 minutes. Scrape the mixture into a large bowl. Wipe out the mixing bowl and whisk. Whip the egg whites with the lemon juice at medium-high speed until soft peaks form. Gradually beat in the remaining 1/4 cup of sugar until glossy. Fold the beaten whites into the yolks. Sift the cocoa over the eggs and fold in until no streaks remain. Spread the batter in a parchment-lined 12-by-15-inch rimmed baking sheet. Bake in the center of the oven for about 30 minutes, until the cake is springy and cooked through. Let cool completely on a rack. Using a 2 2/3-inch round biscuit cutter, stamp out 16 rounds. Invert the baking pan onto a work surface and remove the biscuits.
In a small bowl, sprinkle the gelatin over the water and let stand until softened. In a medium saucepan, bring the milk and 2/3 cup of the heavy cream to a simmer. Pour the liquid into the blender, add the cocoa nibs and let stand for 20 minutes.
Puree the mixture and strain it into the saucepan; bring to a simmer. In a medium heatproof bowl, whisk the egg yolks with the sugar. Gradually whisk in the cocoa nib liquid. Return the mixture to the saucepan and cook over low heat, whisking constantly, until thickened, about 4 minutes. Off the heat, whisk in the gelatin until melted. Pour the mixture into a clean bowl and refrigerate until chilled, about 2 hours. Whip the remaining 1 cup of heavy cream to soft peaks and fold it into the chilled cocoa nib mixture. Refrigerate until firm, about 30 minutes.
Preheat the oven to 350°. In a food processor, pulse the flour with 3/4 cup of the hazelnuts until the nuts are finely ground. Add the sugar and salt and pulse to combine. Add the butter and process until incorporated. Add the remaining 1/2 cup of hazelnuts and pulse to coarsely chop. Press the mixture into small clumps onto a parchment-lined baking sheet and freeze just until firm, 10 minutes. Bake in the oven for about 30 minutes, until golden and fragrant. Let cool completely, then break up into smaller clumps.
Sprinkle 2 tablespoons of the hazelnut crumble into each of 8 wide glasses. Top with 2 rounded tablespoons of the cocoa nib mousse, 1 chocolate biscuit, 2 rounded tablespoons of the chocolate cream and 2 more tablespoons of the hazelnut crumble. Repeat the process with the remaining mousse, biscuits, cream and hazelnut crumble. Serve right away.