Impossible to resist these soft and crumbly cookies that melt in your mouth.

Set the oven rack in the center of the oven. Preheat the oven to 350°F(180°C ).
In the bowl of a stand mixer on medium speed or in a food processor, beat the butter, brown sugar, salt, egg yolk and vanilla until smooth. Scrape the bottom of the bowl once during this step.
Add the flour and mix until no fine particles remain at the bottom of the bowl. Remove the dough from the bowl and press with your hands to shape in a ball. If dough is too soft to shape into balls refrigerate for about 30 minutes.
Place walnuts in a small bowl.

Shape the dough into about 1 inch balls, coat with chopped walnuts and place on an ungreased cookie sheet, spacing them 2 inches apart.
Make a hole in the middle of each cookie with your thumb and then fill with jam using a teaspoon.

Bake for 15 Minutes, one sheet at a time. Let stand 3 Minutes, remove from pan with a thin spatula, place on a wire rack and let cool. It is normal for nuts to fall off during cooking.

Store in an airtight metal box preferably.