Preheat the oven to 350°F. Spread the nuts on a rimmed baking sheet in a single layer. Bake for 15 to 20 minutes, stirring and checking the nuts a few times, until they are golden brown. Let the nuts cool completely, then, if they still have the skins on, rub the nuts together in a dish towel or pour them into a coarse mesh strainer and rub them against the mesh until as much skin flakes off as much as possible. Set a 1/4 cup of the nuts aside for garnish and put the remaining 1 cup in a blender or food processor and process to the smoothest possible paste you can manage, scraping the container as necessary.

Add the chopped chocolate to a large heat-proof bowl and set it aside. In a another medium-sized heat-proof bowl, whisk the eggs together until no streaks of white remain.

In a medium saucepan, stir together the milk, sugar, salt and the hazelnut paste and heat it until it begins to steam and is just about to simmer. Using a ladle or a measuring cup, scoop out about a ½ cup of the hot milk and, while whisking the eggs constantly, gradually stream it into the eggs. Then add the warm egg mixture back into the saucepan and, while stirring constantly, cook over low heat until the mixture is slightly thickened (it should reach 165ºF on an instant-read thermometer). Pour the hot egg mixture through a fine-mesh sieve into the bowl with the chocolate and stir until the mixture is smooth. Let it cool on the counter until it’s not steaming hot anymore, about 15 minutes, then add the mascarpone, Frangelico and vanilla and stir until completely combined. Refrigerate until the mixture is cool, about 45 minutes.

Remove the cooled chocolate mixture and stir it to make sure it’s not too solid. Add the crème fraîche and ½ cup cream to a medium bowl (or the bowl of a stand mixer) and whip it until it begins to stiffen and hold a soft peak. Add the whipped cream to the bowl with the chocolate mixture and gently fold it in until no streaks remain. Pour the mousse into individual serving cups and chill it for at least 4 hours or up to 2 days. When ready to serve, whip another ½ cup cream to medium peaks and add a dollop to the top of each serving of mousse. Chop the reserved nuts and sprinkle them over the cream before serving.