Heat oven to 350°.
In a small bowl, whisk together the flour and salt.
In a medium bowl, whisk together the egg whites, egg, sugar, and vanilla extract. Add the hazelnuts and stir to combine. Refrigerate the mixture for 30 to 45 minutes.
Stir the flour mixture into the cooled egg mixture until it forms a thin, runny dough.
Cover two baking sheets with parchment paper. Place a teaspoon of the dough onto a baking sheet and, with the back of a spoon, gently spread it into a 1- to-1 1/2-inch circle. Repeat with the remaining dough, spacing the cookies 2 inches apart.
Bake the cookies until the edges are golden brown, 15 to 18 minutes. Let them cool on the baking sheet for 5 minutes, then carefully transfer them to a wire rack.
Make sandwiches by spreading 1/2 teaspoon of the chocolate-hazelnut spread on one cookie, then placing a cookie on top.