Preheat oven to 350°F. Line a baking tray with parchment paper and set aside.
In a large bowl, beat egg and ¾ cup Nutella with a hand mixer for 60-90 seconds, until smooth and glossy. Add flour and mix to combine and form a dough.
Cover and refrigerate for 15 minutes.
Use a 1 ¼ tablespoon cookie scoop (size #50) to scoop dough and roll it into 1-inch balls. Coat each dough ball evenly with chopped hazelnuts and place on the lined baking tray about 2 inches apart.
Make an indentation in the center of each ball with your thumb or the back of a round teaspoon.
Add Nutella to a zip top bag or piping bag. Snip the corner and pipe the Nutella into the indentations. Alternatively, spoon Nutella into the cookies.
Bake for 10 minutes. Cool for 10 minutes before serving. Enjoy warm from the oven or completely cooled!
Notes
Keep Nutella cookies in an airtight container on the counter for up to 3 days.
Freeze cookies on a baking sheet before transferring them to a zippered bag. Thaw before enjoying!