Make the caramel by boiling the unopened can of sweetened condensed milk: place the can in a large pot and cover completely with water, then simmer gently for 4–5 hours, keeping the can submerged by adding water as needed; allow the can to cool completely before opening.
Soak the sheet gelatin in cold water according to package instructions until softened, then squeeze out excess water and set aside.
Heat the heavy cream in a saucepan over medium-low heat until warm but not boiling.
Add the softened gelatin to the warm cream and whisk until the gelatin is fully dissolved.
Stir about 50 g (roughly 1 ¾ oz) of the prepared caramel into the cream mixture until evenly combined.
Divide the mixture evenly among 3 serving glasses or ramekins, then refrigerate for at least 3 hours, or until set.
To serve, sprinkle the chopped toasted hazelnuts on top of each panna cotta and drizzle with additional caramel.