Whip together the Nutella and eggs on medium speed for about 3 minutes.
Slowly add in half of the sifted flour mixing until combined, then add in the second half.
Once fully mixed, scrape the sides of the bowl and push all of the dough to the center, cover the dough tightly with plastic wrap and chill in the refrigerator for at least 25 minutes.
While the dough is chilling you can preheat your oven to 350 degrees F.
Prepare one or two cookie sheets by lining it with parchment paper.
Once the dough has chilled for at least 25 minutes you can remove it from the fridge.
Scoop about 1.5 Tablespoons of the dough into your hand and roll into a ball. Place the ball onto the prepared baking sheet, repeat this step until all of the dough has been rolled into balls. Keep the dough balls a few inches away from each other on the sheet, you may need a second cookie sheet.
Make a deep indentation into the cookie, to do this you could use your thumb, or the end of a rolling pin, or a kitchen utensil. The indentation should be deep but not all the way through, and not too wide (see pictures).
Bake the cookies at 350 degrees F for about 12 to 15 minutes. The cookies will be slightly soft but will continue to firm while cooling.
While the cookies are cooling you can decide which filling to use. You can either fill with plain Nutella or you can make a Nutella buttercream. To make the buttercream, add all of the filling ingredients to a bowl and mix with an electric hand mixer or stand mixer for 5 minutes until light and fluffy.
Add your choice of filling (Nutella or Nutella Buttercream) to a pastry bag fitted with a star shaped piping tip.
Once the cookies are completely cooled, pipe the filling into the indentation, swirling as you go up. Sprinkle the top with chopped hazelnuts.