Preheat oven to 350. Spray a 13x9 pan with cooking spray.
In a large bowl, place cake mix, melted butter and egg. Mix for 1-2 mins. on medium speed until crumbly. Set aside 1 cup of crumb mixture.
Press firmily the remaining crumb mixture into the bottom of the pan. Bake 15 mins.
In a medium saucepan, combine 1 cup of chocolate chips and sweetened condensed milk. Warm over low heat and stir until smooth. Remove from heat after its smooth and stir in vanilla. Spread over hot crust.
If using nuts, stir nuts and remaining 1 cup chocolate chips into reserved crumb mixture. Sprinkle over chocolate filling.
Bake 25-28 mins. or until the topping is firm to the touch. Cool completely before cutting into bars.