In a medium saucepan, combine chocolate, Nutella, cocoa powder, 1/2 cup water, and 1/4 cup sugar. Cook over medium heat, stirring, until chocolate melts and ingredients are blended, 4 minutes. Remove from heat; set aside.
In a large bowl with a mixer on medium speed, beat egg yolks and 1 tbsp. sugar until pale and slightly frothy, 4 to 5 minutes. Reduce speed to low and add 5 tbsp. chocolate mixture, 1 tbsp. at a time. Stream in remaining chocolate mixture while beating. Increase speed to medium and beat until well blended and a little thicker, about 2 minutes. Beat in vanilla.
In a clean bowl using clean beaters, beat egg whites and 1 tbsp. sugar to soft peaks. Gently fold whites into chocolate.
In bowl used for egg whites, beat cream and the remaining 1/4 cup sugar on medium-high speed to soft peaks. Gently fold a bit less than half of whipped cream into mousse until no streaks remain. Spoon mousse into glasses and chill, covered, until set, about 2 hours.
To serve, beat remaining softly whipped cream just until stiff. Pipe onto chocolate mousse through a pastry bag fitted with a wide star tip. Sprinkle with hazelnuts.
Note: This dessert contains raw eggs.
Make ahead: Through step 4, up to 1 day.