In the bowl of a standing, electric mixer, combine the butter, combine egg yolks, brown sugar, and white sugar. Beat until light and fluffy, approximately 2 minutes.
Add milk, vanilla extract, flour, and salt. Beat until combined.
Chop nuts and put them on a plate large enough to roll your cookies.
Beat egg whites with a fork in a small bowl until airy (2 minutes).
Scoop two tablespoons of dough and roll it into a ball. Coat the ball in egg whites and then roll in chopped hazelnuts. Gently press your thumb into the center of the cookie until indented. Place on a parchment paper lined baking sheet.
Bake for 14-16 minutes, or until lightly browned on the edges. If indentation has disappeared, press gently with the end or a wooden spoon while hot. Cool completely.
Pipe or spoon Nutella topping into the indented center. Store in a covered container in a cool place.
This recipe was calculated using the exact brands and measurements I used to make this recipe. If you are following a strict diet please note changing anything will cause the nutritional info to change. My calculations are intended as a guide only.