In large bowl, combine flour and sugar. With pastry blender or 2 knives, use in scissor fashion, cut in butter until mixture resembles coarse crumbs. Combine egg yolk and vanilla. Add to crumb mixture, stirring until dough holds together. Press into 15x20-inch jellyroll pan. Bake 15-20 minutes at 350F until lightly browned.
Remove from oven and immediately place chocolate bars over hot cookies. Let stand until chocolate is softened. Spread chocolate evenly over dough, then sprinkle with walnuts. Cool completely on wire rack. Cut into bars.