How to Make It
Oil a baking sheet and line eight 1/2-cup ramekins or custard cups with plastic wrap, leaving 3 inches of overhang. Preheat the oven to 350°. Put the hazelnuts on a rimmed baking sheet and toast for about 10 minutes, or until richly browned. Transfer to a kitchen towel and let cool. Rub the nuts together in the towel to remove the skins; finely chop the hazelnuts.
Put 1/2 cup of the sugar in a small saucepan. Stir the water into the sugar and bring the mixture to a boil over moderate heat. Simmer this sugar syrup, brushing down the sides of the saucepan with a wet pastry brush, until a deep brown caramel forms, about 5 minutes. Remove the pan from the heat and, using a wooden spoon, stir in the hazelnuts. Immediately scrape the caramel-and-hazelnut mixture onto the oiled baking sheet and let cool until hard.
Break this praline into small pieces and transfer to a food processor. Grind to a coarse powder.
In a medium stainless steel bowl, whip 1 cup of the heavy cream until it holds firm peaks and refrigerate. In a large stainless steel bowl, beat the egg whites until they hold firm and glossy peaks, gradually adding the remaining 1/2 cup of sugar. Using a large rubber spatula or a wooden spoon, fold in the ground praline, followed by the whipped cream. Spoon the semifreddo into the prepared ramekins, pressing with the spoon to remove any air pockets. Cover with the overhanging plastic and freeze until firm, about 2 hours.
In a small saucepan, bring the remaining 1/3 cup of heavy cream to a boil over moderate heat. Remove the pan from the heat and stir in the chopped chocolate until melted. Stir in the vanilla extract and a pinch of salt and let cool slightly. Refrigerate the chocolate sauce until thick, about 1 hour.
Unwrap the semifreddo and unmold onto dessert plates. Drizzle the sauce around the plates and serve.