Preheat oven to 350 degrees. Lightly spray a 9 x 13" pan with non-stick baking spray. Set aside.
In a bowl, combine the cake mix, melted butter, and milk. Mix until completely combined. Spread 1/2 of the batter into an even layer on the bottom of the prepared pan. Make sure to reserve the remainder of the batter. Bake the bottom crust for 15 minutes at 350 degrees.
Remove from oven. While bottom crust is still hot, pour the warmed peanut butter evenly over the crust. Next, sprinkle with the chocolate and peanut butter chips. Using a teaspoon, evenly spoon small dollops of the reserved batter over the chocolate chip layer. Bake again for an additional 15 minutes.
Remove from oven and let cool completely before cutting into squares. Store any leftovers in an airtight container.