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 warm, evenly sprinkle on the chocolate chips and then the M&M's. Using a teaspoon, evenly spoon small dollops of the reserved batter over the M&M 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.
Notes
Storage: Store the cooled bars in an air tight container at room temperature. Bars will keep for up to 5 days at room temperature.