Preheat over to 350º. Toast walnuts by spreading evenly on ungreased cookie sheet. Stirring occasionally, bake 5-8 min or until darker in color. Cool completely on baking sheet. Process walnuts in a food processor until finely ground.
In a large bowl, combine flour, baking soda, and salt. Using a pastry blender or 2 knives, cut butter into flour until mixture resembles coarse meal. Add ground walnuts, sugar, egg yolk, an vanilla; stir until a soft dough forms. Shape dough into 1” balls and place 2” apart on a greased baking sheet. Press balls with back of fork in a crisscross pattern to form ¼” thick cookies. Bake 15-20 min or until edges are lightly browned. Transfer coolies to a wire rack. Dust warm cookies with confectioners sugar, cool completely. Dust with confectioners sugar again. Store in an airtight container.