Preheat oven to 350°F

Add the butter and sugar to the bowl of your electric mixer fitted with blade attachment

Beat on high until butter is smooth and mixture is fluffy, about 2 minutes

Separate the egg yolk and egg white, set the white aside in a small bowl

Add the yolk to the mixing bowl, beat

Add the vanilla extract, mix

Slowly mix in the flour and salt

Mix until flower is fully combined*

Use clean hands to work the dough into 12 evenly sized, round balls

Add the water to the egg white, whisk with a fork until mixture is smooth

Pour the chopped hazelnuts onto a plate and spread out in an even layer

Dip each dough ball first in the egg white mixture and then in the hazelnuts to coat half

Lay the cookies on a parchment lined baking sheet, evenly spaced with the hazelnut covered side up

Using clean hands, press your thumb into the middle of the top of each dough ball, press about halfway into the ball to form a shallow hole**

Bake for 14-16 minutes until the bottom of the cookies are lightly browned***

Remove from oven and let cool 5 minutes

Fill the thumbprint hole on the top of each cookie with about 1 tbsp of Nutella

Transfer to a wire rack to cool