Heat oven to 350. Line a 12-cup mini muffin pan with paper or foil liners.
Put the Nutella and egg in a medium bowl and whisk until smooth and well blended. Add the flour and whisk until blended.
Spoon the batter into the prepared muffin tins (about 3/4 full) and sprinkle with the chopped hazelnuts.
Bake until a pick comes out with wet, gooey crumbs, 11-12 minutes. Set on a rack to cool completely. Serve immediately or cover and store at room temp. for up to 3 days.