Preheat the oven to 170°C.
Line 12 cups of a mini-muffin pan with paper liners.
In a medium bowl, stir together the Nutella and egg until combined and smooth. Mix in the flour.
Spoon the batter into the muffin cups, filling them about three-quarters full.
Sprinkle with the hazelnuts.
Bake until a wooden skewer inserted into the centre comes out with wet, gooey crumbs, about 12 minutes.
Set on a rack to cool completely.
Serve immediately or store in an airtight container at room temperature for up to 3 days.
