Melt butter in a microwaveable bowl and set aside to cool. Preheat oven to 375 degrees and line a baking sheet with parchment paper.
In a medium bowl, whisk together flour and baking soda.
In a large bowl, combine butter, both sugars, vanilla, and salt. Beat with stand or handheld mixer until well combined.
Add egg and egg yolk. Scrape down sides of bowl as necessary.
Add in flour mixture and continue mixing until the batter is incorporated. Mix in chocolate chips and M&Ms and gently stir.
Using a large spoon or ice cream scoop, measure out 2 tablespoonfuls of cookie dough into balls and place on cookie sheet about 2 inches apart. Top each cookie dough ball with additional M&Ms.
Bake for 8 to 9 minutes until the edges are just lightly browned. Cookies will look slightly underdone but the cookie will continue to bake as it cools.
Allow cookies to cool completely before moving.