Preheat oven to 350. Reserve 2 tablespoons butter. In large mixer bowl, beat remaining butter and sugar until fluffy. Add flour and cocoa; mix well. With floured hands, pressed into greased 13"x9" pan. Bake 15 minutes.
Meanwhile, in medium saucepan, combine reserved butter and sweetened condensed milk; cook and stir until mixture thickens slightly, about 15 minutes. Remove from heat; stir in vanilla. Pour over crust. Bake 10 to 15 minutes longer or until golden brown. Remove from oven; immediately top with chips. Let stand 1 minute; spread while warm. Top with nuts. Cool. Cut into bars. Store covered at room temperature.