Preheat oven to 350° degrees F. Line a 13x9 rectangular light metal baking dish with foil, extending the sides of the foil over the edges of the pan. Spray the foil with cooking spray; set aside briefly.
In a large bowl, combine the cake mix, egg, and oil (or butter if using) with a rubber spatula or wooden spoon until a soft dough comes together. Press the dough evenly into the bottom of the prepared pan. Bake the dough base for 12 minutes. Remove from the oven but keep oven on.
Liberally and evenly top the dough base with the chopped cookies, white chocolate chips, and rainbow jimmies. Drizzle the sweetened condensed milk evenly over the top. Return to the oven and continue baking for 20-22 minutes or until the top is very light golden brown and the bars appear set.
Remove from the oven and allow to cool completely in the pan until room temperature. I like to either let set at room temperature for at least 2 hours, or refrigerate for 1 hour, to really set the bars for clean slices. You could even cover and let set overnight if you'd prefer.