Cream butter and sugar. Beat in flour, egg yolk and vanilla until well blended.
Spread evenly in a well greased 15 1/2" x10 1/2" jelly roll pan. Tip: you can cover dough with wax paper and pat it out. Then use another flat square pan and spread with that.
Brush top with very slightly beaten egg white. Tip: I just tilt the pan and let egg white run all over the dough. Sprinkle with chopped nuts.
Bake 1 hr and 10 minute or until golden. While baking check to see if the top is puffing up. If it is use a toothpick to prick in several places.
When done remove from oven and IMMEDIATELY cut into 50 bars and cool on rack.