Sift and measure flour. Sift with sugar and salt into bowl. Cut in butter until mixture looks like corn meal. A pastry blender is just right for this. Stir in vanilla and egg yolks. Knead for a few minutes, being careful not to use any extra flour. Use 1 tablespoon flour for this purpose. Cut dough into 100 pieces; these are shaped into marble-sized balls. Place in single layers on waxed paper-lined pan. Cover with more waxed paper. Refrigerate overnight.
At this time, you can make the walnut filling: Combine egg whites and powdered sugar. Add the ground nuts to egg mixture. Beat vigorously for about 5 minutes. Place filling in refrigerator.
Remove only 5 balls of dough from refrigerator at one time. Roll these out paper thin, using as little flour as possible. Place a level teaspoon of filling in center. Enclose by loosely overlapping 2 sides. Secure ends with a light twist. Place 1/2 inch apart on ungreased cookie sheet. Bake at 350° for 20 to 25 minutes, or until lightly browned. Remove carefully. Cool and sprinkle with powdered sugar.