1Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. In large bowl, mix butter, brown sugar, vanilla and egg yolk. Stir in Bisquick mix. Press in pan.
2Bake 20 to 25 minutes or until very light brown (crust will be soft). Immediately sprinkle chocolate chips on hot crust; let stand about 5 minutes or until chocolate is soft. Spread chocolate evenly; sprinkle with nuts. Cool 30 minutes in pan on cooling rack. For bars, cut into 8 rows by 4 rows.