In a medium bowl, cream together the butter and brown sugar until smooth. Beat in the egg yolks one at a time. gradually mix in the flour. Cover dough and chill until firm, about 2 hours.
Preheat the oven to 325 degrees F (165 degrees C). Line cookie sheets with parchment paper or aluminum foil. Roll dough into balls a little smaller than walnuts. Dip the balls in egg white, then roll them in the chopped walnuts. Place cookies 1 inch apart onto the prepared cookie sheets. Press thumb into middle of each ball to create a divot.
Bake for 12 to 15 minutes in the preheated oven, or until edges are golden. Remove from oven and repress divots, fill the divots with 1/2 teaspoon of raspberry jam. Remove cookies to cool on wire racks.