Preheat the oven to 350°F. Coat a 9x13 glass baking dish with cooking spray. Set aside.
In the bowl of a stand mixer using the paddle attachment or an electric hand mixer in a large mixing bowl, stir together the cake mix, pudding mix, water, vegetable oil and eggs until almost smooth.
Fold in 1 cup of the M&M's. Do not over fold or the colors will start to bleed.
Evenly pat the batter into prepared baking dish. Top with the remaining 1/2 cup M&M's and lightly press down.
Bake the cookie bars 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.
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