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Start by melting butter and setting butter aside to cool completely.
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Preheat oven to 375 degrees and line baking sheet with parchment paper.
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In a medium bowl, whisk together flour and baking soda.
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In the bowl of a stand mixer with paddle attachment, combine melted and cooled butter, both sugars, vanilla and salt. Beat on medium speed until well combined, about 2 minutes.
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Add egg and egg yolk, scraping down sides of bowl as necessary.
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Add in flour mixture and continue mixing until incorporated.
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Add in chocolate chips and M&M's and give a final stir.
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Measure 1/4 cup of cookie dough and shape into a ball.
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Dough will be soft, but should be firm enough to shape into balls.
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Place cookie dough on baking sheet and top with additional M&M's.
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Bake 8-9 minutes, just until edges are a light golden brown.
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Cookie should look slightly under done.
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Don't over bake as cookie will continue baking as it cools.