Preheat oven to 275°. Grease a 10x15x1-inch jelly roll pan. In a mixing bowl, combine flour, sugar, butter, vanilla, and egg yolk. Beat ingredients at low speed of electric mixer until blended; increase speed to medium and beat until well mixed. Scrape bowl occasionally.
Pat dough evenly into the greased pan. In a cup with fork, beat egg white slightly; brush over dough then sprinkle with chopped walnuts.
Bake for 1 hour and 10 minutes, or until golden. Immediately cut lengthwise into 4 strips, then cut each strip crosswise into 12 pieces, to make
48 bars. Remove bars from pan to cool on racks. Store in tightly covered container for up to 1 week.
Calories per bar: 76