Heat oven to 350°
Mix butter, sugar, egg yolk, and vanilla.
Measure flour by dipping method or by sifting.
Stir in flour and salt until dough is well blended.
Spread in a rectangle about 13x10" on greased baking sheet, leaving about 1" all around edge of baking sheet.
Bake 20 to 25 min., or until nicely browned.
Crust will still be soft.
Remove from oven.
Immediately place separated squares of chocolate on top.
Let stand until soft; spread evenly over entire surface.
Sprinkle with nuts.
Cut in small squares while warm.