Preheat oven to 350 degrees F. Line a 13x9" rectangular baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray.

In a large bowl, combine the cake mix, melted butter, and egg with a rubber spatula until a soft but thick dough forms. Press the dough evenly into the prepared pan and bake for 12 minutes. Remove from the oven, but keep the oven on.

Immediately top the crust evenly with the chopped Oreo cookies, the candy bars, and both types of chocolate chips. Drizzle the sweetened condensed milk evenly over the bars. Return to the oven and bake for 18-20 minutes until set and bubbly. Cool completely before cutting into bars.