Preheat oven to 350 degrees. Coat a 9x13 glass baking dish with cooking spray.
On the bowl of a stand mixer using the paddle attachment or an electric hand mixer in a large mixing bowl, stir together cake mix, pudding mix, water, vegetable oil and eggs.
Fold in 1 cup M&M's. Do not over fold or the colors will start to bleed.
Evenly pat batter into prepared baking dish. Top with remaining 1/2 cup M&M's and lightly press down.
Bake for 25-30 minutes or until tops are uniformly light brown and it passes the toothpick test.
Remove and allow to cool. Cut into squares and enjoy your M&M Cookie Bars!
If you've tried these, come back and let us know how they were!