Start by blooming the gelatin. Place the sheet gelatin in a small bowl and cover it with cold water. Let it sit for about 5 minutes until it softens.
If you haven't already toasted your hazelnuts, do so now. Preheat the oven to 180°C (350°F) and spread the hazelnuts on a baking sheet. Toast for about 10 minutes, or until they are golden and fragrant. Let them cool, then roughly chop them.
In a small saucepan, combine the heavy cream and sweetened condensed milk over medium heat. Stir occasionally until the mixture is just about to boil. Remove it from the heat.
Once the cream mixture is hot, add the bloomed gelatin to the saucepan. Stir until the gelatin is completely dissolved.
Stir in the toasted hazelnuts, allowing their nutty flavor to infuse the cream.
Pour the mixture into your serving cups or molds. Cover with plastic wrap and refrigerate for at least 4 hours, or until set.
While your panna cotta is chilling, prepare the caramel sauce if you’re making it from scratch. In a saucepan, heat the caramel sauce over low heat until it’s smooth and pourable.
Once the panna cotta has set, carefully run a knife around the edges of the molds if necessary and invert onto plates. Drizzle with the warm caramel sauce, and garnish with extra toasted hazelnuts.