In a small bowl, sprinkle the gelatin over 1 tablespoon of water; let stand until the gelatin is softened, 5 minutes. In a medium saucepan, bring the cream and 6 tablespoons of the sugar to a simmer; remove from the heat. Stir in the gelatin until dissolved. Add the gianduja and salt to the hot cream and let stand until melted, about 3 minutes; whisk until smooth. Scrape the mixture into a glass bowl, press a sheet of plastic wrap directly onto the surface, and let stand until cooled, about 1 hour.

Meanwhile, spray an 8-inch springform pan with nonstick cooking spray. In a food processor, pulse the cookies until fine; transfer to a bowl. Add the melted butter and the remaining tablespoon of sugar and stir until coated. Press the crumbs over the bottom of the springform pan and refrigerate the crust until firm, about 30 minutes.

Preheat the oven to 325 degrees. Spread the hazelnuts on a baking sheet and toast for 12 minutes, until fragrant. Let cool.

Pour the gianduja filling over the crust and scatter the toasted hazelnuts over the surface (they will sink and then float to the surface). Refrigerate the tart uncovered until firm, about 6 hours.

Run a small hot knife around the edge of the tart. Remove the ring from the pan, cut the tart into wedges and serve.

Look for gianduja bars at specialty-food shops.