Heat oven to 350 degrees. Lightly butter 13x9-inch pan.
In mixer bowl, beat butter, brown sugar, egg, and vanilla until creamy.
Spread into prepared pan.
Bake for 25 minutes or until lightly browned.
Immediately sprinkle with chocolate chips and let stand 5 minutes to allow chips to melt. Spread chocolate over cookie base.
Sprinkle with walnuts.
Cool; cut into bars.