Crust
Vegetable oil cooking spray
3 (5 inches long by 3/4-inch wide) plain or almond biscotti (about 4 ounces) halved
1/4 cup toasted hazelnuts, see Cook's Note
1/2 stick (4 tablespoons) unsalted butter, melted

Filling
8 egg yolks, at room temperature
1/2 cup sugar, plus 1/4 cup
1 teaspoon pure vanilla extract
1/8 teaspoon fine sea salt
1 cup heavy cream
3/4 cup chocolate-hazelnut spread at room temperature (recommended: Nutella)
Special equipment: a 3 by 15-inch piece of parchment paper, a 7 1/2 by 12-inch piece of parchment paper