Special equipment: 3 by 15-inch piece of parchment paper, a 7 1/2 by 12-inch piece of parchment paper.
For the crust: Put an oven rack in the center of the oven. Preheat the oven to 350 degrees F. Spray a 9 by 5 by 3-inch nonstick loaf pan with vegetable oil cooking spray. Using 2 pieces of parchment paper, line the pan, allowing the excess to hang over the ends and sides.
In a food processor, blend the biscotti and hazelnuts together until finely ground. Add the melted butter and pulse until the crumbs are moistened. Using a flexible spatula, press the crumb mixture into the bottom of the prepared pan. Bake for 8 to 10 minutes until the edges of the crust are golden. Cool the pan on a wire rack for 20 minutes.
For the filling: In a medium stainless steel or glass bowl, whisk together the egg yolks, 1/2 cup sugar, vanilla extract, and salt until smooth. Put the bowl over a pan of simmering water, making sure the bottom of the bowl does not touch the water. Whisk until the egg mixture is pale, thick and creamy, and an instant-read thermometer registers 160 degrees F, about 10 to 15 minutes. Put the bowl into a larger bowl of iced water to cool completely.
In another medium bowl, using an electric mixer, beat the cream until thick. Add the remaining 1/4 cup sugar and beat until the cream holds stiff peaks. Mix 1/4 of the cream into the cooled custard. Using a spatula, gently fold the remaining cream into the custard. Drop spoonfuls of the chocolate-hazelnut spread over the custard mixture and gently fold until just incorporated but still chunky. Spoon the mixture onto the prepared crust. Fold the overhanging parchment paper over the custard and freeze for at least 8 hours or up to 3 days.
Run a thin-bladed spatula around the semifreddo between the parchment paper and the pan. Unfold the parchment paper and invert the semifreddo onto a platter. Peel off the paper and cut into 1-inch thick slices. Serve on chilled plates.
To toast the hazelnuts, arrange in a single layer on a baking sheet. Bake in a preheated 350 degrees F oven for 8 to 10 minutes until lightly toasted. Cool completely before using.