1. Make crust and toast nuts: Put oven racks in upper and lower thirds of oven and preheat oven to 350 degrees F. Invert bottom of springform pan (so that turned-up edge is underneath for easier removal of tart) and close side of pan onto bottom.
2. Stir together crumbs, butter, and 1/8 teaspoon salt in a bowl, then press onto bottom of springform pan. Bake crust in lower third of oven 7 minutes, then cool crust in springform pan on a rack.
3. While crust bakes, toast hazelnuts in a shallow baking pan in upper third of oven, stirring once, until golden, about 7 minutes. Transfer to a plate and cool to room temperature.
4. Make filling while crust cools: Bring cream to a boil in a 1- to 2-quart saucepan, then pour over chocolate in a heatproof bowl, whisking until chocolate is melted and smooth. Whisk in chocolate-hazelnut spread until combined, then whisk in nuts and remaining 1/8 teaspoon salt.
5. Pour filling onto cooled crust and put in freezer until just set, about 25 minutes. (Tart should not be frozen.) Remove side of pan before serving tart.

Yield: 10 servings