Line a baking sheet with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment whisk together the egg and 3/4 cup Nutella for about 2 minutes until the mixture is glossy.
Add flour and beat until combined. It's important to measure the flour correctly to avoid adding too much, I prefer using a kitchen scale but if you're using a measuring cup, use a spoon to fill the cup with flour and don't scoop it.
Cover bowl with plastic wrap and let rest in the fridge for 10-15 minutes. Don't leave the dough longer in the fridge or the dough will get cracked when forming the balls.
Preheat the oven to 350°F (177°C).
Using a tablespoon, scoop out dough and form 20-25 cookie balls. Place cookie balls on the baking sheet.
Use your finger or the end of a wooden cooking spoon to make a dent in the center of each cookie.
Fill the hole with Nutella. The easiest way is to use a pastry bag or a Zip Lock bag for this.
Sprinkle chopped hazelnuts on top of the Nutella.
Bake for about 10 minutes. Take the cookies out of the oven and let them cool down on the baking sheet for 10 minutes, then transfer to a cooling rack and let cool completely.
Store the cooled cookies in an airtight container in a cool and dry place. They will keep fresh for about a week.
Recipe updated 12/21 because some readers had problems with the dough being too dry. I reduced the amount of flour from 1 1/4 cups to 1 cup. Please measure the flour correctly using the spoon and level method or use a kitchen scale!