Preheat the oven to 350ºF. Line a 9x13 baking pan with foil and spray with cooking spray.
In a large bowl, combine cake mix, butter and egg. Press mixture evenly into prepared pan. Cover with both chocolate chips and drizzle condensed milk evenly over the top.
Bake approximately 23 minutes or until the top has cooked evenly.
Let cool 5 minutes, then run a knife around the edges to loosen them from the foil. Let cool completely, then lift the foil to remove from the pan and cut into squares.