Preheat oven to 350 degrees Fahrenheit and line a baking sheet with parchment paper.

Cream together the butter and granulated sugar in a large bowl until light and fluffy.

Beat in the egg yolk and vanilla extract.

Gradually stir in the flour and salt until a dough forms.

Roll the dough into 1-inch balls, approximately 24 total.

Dip each ball into the beaten egg white then roll in the chopped hazelnuts to coat.

Place balls 2 inches apart on the prepared baking sheet.

Press a deep indentation into the center of each ball using your thumb or the back of a small spoon.

Bake for 12 minutes or until the edges are lightly golden.

If indentations have puffed up, gently press them down again while the cookies are still warm.

Transfer cookies to a wire rack to cool completely.

Once cooled, fill each indentation with approximately 1 teaspoon of chocolate-hazelnut spread.