1. Preheat the oven to 350°F.
2. Line two baking sheets with parchment paper. In the bowl, cream the butter and confectioners sugar until light and fluffy, 4 to 5 minutes. (you can use a handheld mixer or stand mixture). Add the vanilla extract and mix to combine for about 1 minute. Add the egg whites gradually, mixing until fully incorporated, 2 to 3 minutes. Add the flour and salt and mix just until incorporated, 1 to 2 minutes.
3. Transfer the batter to a 1-gallon ziplock plastic bag. Cut an opening at one of the bottom corners. Pipe the batter into lines 1½ inches long on the baking sheet. You can also make them into round cookies as well if you like.
4. Bake until the cookies are set and the edges are pale golden, 9 to 11 minutes.
5. Remove from the oven and cool completely.
6. When the cookies are cool, turn half of them over and drop 1 tablespoon of the Nutella onto the center of each upside-down cookie. Top with another cookie and press gently so that the ganache runs to the edges. Enjoy!

Recipe Notes: cookies can be stored in air tight container for 3 days.