Preheat the oven to 170 degrees celsius (fan-forced).
Line the baking trays with baking paper and set aside.
Add the plain flour, Nutella and egg to a large bowl and mix together using a spoon or beaters. Mix until the dough is completely combined. If using a Thermomix, mix on Speed 4, 10 seconds. Scrape down the sides of the bowl and repeat until well combined.
Roll tablespoons of the mixture into balls and place onto the prepared trays. Use a fork to lightly press the cookie dough balls down.
Bake for 8 minutes (do NOT overcook). Leave the cookies on the baking trays for 5 minutes before transferring to a wire rack to cool completely.