In a microwave-safe bowl, combine butterscotch chips and 1/3 of the sweetened condensed milk. In another microwave-safe bowl, combine semi-sweet chocolate chips, the remaining sweetened condensed milk, and vanilla. Microwave the chocolate chips for 1 minute on HIGH. Stir until melted. Stir in chopped walnuts. Spread into an aluminum foiled-lined 13"x9' pan. Next, microwave the butterscotch chips on HIGH for 45 seconds. Stir until melted, then spread evenly over the chocolate mixture.