Start by melting butter and setting butter aside to cool completely.

Preheat oven to 375 degrees and line baking sheet with parchment paper.

In a medium bowl, whisk together flour and baking soda.

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.

Add egg and egg yolk, scraping down sides of bowl as necessary.

Add in flour mixture and continue mixing until incorporated.

Add in chocolate chips and M&M's and give a final stir.

Measure 1/4 cup of cookie dough and shape into a ball.

Dough will be soft, but should be firm enough to shape into balls.

Place cookie dough on baking sheet and top with additional M&M's.

Bake 8-9 minutes, just until edges are a light golden brown.

Cookie should look slightly under done.

Don't over bake as cookie will continue baking as it cools.