With a hand or stand mixer, cream together the butter and sugars. Add in the egg, egg yolk and vanilla. Mix together until completely incorporated and smooth.
In a separate bowl, combine the flour, salt and baking soda.
Gradually add the flour mixture to the wet ingredients, stopping to scrape down the sides of the bowl. Batter will become very thick. Stir in the white and semi sweet chocolate chips and M&M's. Cover bowl with plastic wrap and refrigerate dough for at least 30 minutes.
Preheat oven to 350 degrees. Line a baking sheet(s) with a silicone baking mat or parchment paper.
Using a cookie scoop, roll out balls of the refrigerated dough using your hands. Place on prepared baking sheets, arranging 12 cookies to a pan. Bake cookies for 10-12 minutes or until golden brown on the outside and set in the middle. Repeat with the remaining cookie dough.
After baking, let cookies cool on the baking sheet for 5 minutes. Transfer to a cooling rack for the cookies to cool completely to room temperature.Enjoy!
Notes
For additional color, feel free to add a few extra M&M's to the top of each baked cookie right after baking. Press into the soft, hot cookie for the M&M's to adhere to the cookie.Storage: Store any leftover cookies at room temperature in an airtight container. Cookies will keep for 3-4 days at room temperature.Freezing: Baked cookies can be frozen. Store in a freezer bag or container. Cookies will keep for up to 3 months frozen.Freezing Cookie Dough: Fully mixed cookie dough can be frozen. Be sure to store in an airtight container or freezer bag. Dough will keep for up to 2 months frozen. When ready to bake, let dough thaw to room temperature before rolling into cookies. Bake as directed.