Pre heat oven to 350 degrees. In mixing bowl, beat butter, brown sugar, egg, and vanilla until creamy. Stir in flour, salt, and 1/2 cup of walnuts. Spread into lightly buttered 9x13" pan. Bake 25 mins. or until lightly browned. Remove from oven and immediately sprinkle chocolate chips on top and let stand 5 mins., spread melted chocolate over cookie base. Sprinkle with remaining walnuts and press lightly onto cookie base. Cool, cut into bars.