In the bowl of an electric mixer, cream together the butter and sugar until smooth (3-5 minutes). Scrape the bowl when needed. Then beat in the egg yolks, vanilla, and salt. Finally, beat in the flour until just combined. Cover and chill for about 1 hour.
Preheat your oven to 375 degrees F. Line cookie sheets with parchment paper and set to the side.
Shape the dough into 1-inch round balls. Roll the balls in the egg whites and then in the chopped hazelnuts. Place the balls 1 inch apart on the prepared cookie sheets. Then press your thumb into the center of each ball.
Bake for 10-12 minutes or until lightly browned. If the center puffs during baking, repress it with a spoon. Cool the cookies on a wire rack. In a microwave-safe bowl, microwave the Nutella for about 20 seconds. Fill the center of your cookies with the Nutella. Pack and store them in an air-tight container.
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