Add sugar, butter, and milk to a large saucepan and bring to a roiling boil over medium heat, stirring often. Let it boil for 1 minute, stirring constantly.
Remove from heat and immediately stir in the Nutella, cocoa powder, vanilla, and salt until well combined. Gently stir in the oats until well coated.
Scoop or drop by the spoonful onto lined baking sheets and allow them to set at room temperature until cooled and hardened, about 1 hour.
Notes
Store in an airtight container at room temperature for up to 4 days.