Preheat oven to 350*. Line a 9x13 pan with foil. Melt butter. In a large bowl combine butter, cake mix, egg and vanilla. Blend with a mixer for 1-2 min. or until crumbly. Put 1 cup of the mix into a medium bowl; stir in 1 cup of the chips, and the nuts; set aside.
Press remaining mixture into pan and bake 15 min. In a small saucepan, melt remaining chips and sweetened condensed milk on low heat 1-2 min or until smooth.
Pour over crust. Top with nut mixture, bake an additional 20-25 min, or until topping if firm to the touch. Let stand 30-40 min, or until completely cooled. Remove foil from pan and cut into bars..Enjoy!