Place 1/2 cup milk in a shallow bowl and sprinkle with the gelatin powder. Let soften for 5 minutes.
While gelatin is softening, pour the rest of the milk into a saucepan on medium heat. Add Nutella and stir until Nutella melts and blends with the milk. Add the milk/gelatin mixture and stir until gelatin dissolves. Remove from the heat and stir in the vanilla and a pinch of salt.
Pour into 6 ramekins (each will hold a 1/2 cup). Chill in refrigerator for at least 2 hours, but preferably overnight.
While the panna cotta is chilling, make the candied hazelnuts. Preheat oven to 325. Line a baking sheet with parchment paper or coat with cooking spray. Combine hazelnuts with agave nectar, brown sugar, and a pinch of salt in a bowl. Spread in a single layer on the baking sheet, breaking up clumps. Bake for about 15 minutes. Remove from oven and cool completely on the baking sheet. Store in an airtight container.
Must chill for at least 2 hours, preferably overnight.