Preheat the oven to 350 F degrees. Line a baking sheet with parchment paper; set aside.
In a medium bowl combine the flour and salt together.
In the bowl of your mixer add the butter and brown sugar then mix until light and fluffy for a couple minutes. Add the egg yolk, vanilla extract and continue mixing until combined. Add the flour mixture and mix until well incorporated.
In a small bowl beat the egg white with a fork. In another shallow bowl place the finely chopped walnut.
Shape the dough into 1 inch balls. It might be easier to use a small ice cream scoop to ensure that each ball is of equal size. Roll each ball in the egg white, then roll them in walnuts making sure to coat them well. Place the balls on the prepared cookie sheet and press a deep indentation in the center of each ball with your thumb. I find it easier to use a washed and dried wine cork.
Bake the cookies for 8 minutes or until set. Remove cookies from the oven and fill each indentation with the jam, use about 1/4 tsp per cookie. Return the cookies to the oven and bake for another 8 to 10 minutes until lightly browned.
Place cookies on wire racks to cool completely.
If you use salted butter, there is no need to add additional salt to the recipe.This recipe will yield about 24 cookies.Feel free to add whatever jam or jelly you prefer. Pecans can also be used instead of walnuts.To freeze, place cookies in an airtight container and place in the freezer for up to 3 months.Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.