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Preheat oven to 350 degrees.
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Line a 9x13 baking dish with foil, leave some overhang to make removal easier.
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Spray foil generously with nonstick cooking spray.
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In the bowl of a stand mixer, beat butter, granulated sugar, and brown sugar together until fluffy.
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Add in eggs and vanilla extract and continue to beat.
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Lower speed of mixer and add flour, baking soda and salt, and mix until combined.
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Add M&M's and mini chocolate chips and give a final stir, reserving some M&M's and mini chocolate chips for topping cookie bars.
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Transfer cookie dough to prepared baking dish.
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Press additional M&M'S and mini chocolate chips into top of cookie dough.
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Bake 30 to 35 minutes on oven rack in lower third of oven to prevent over browning.
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Let cool completely and cut.