Preheat oven to 350 degrees Fahrenheit. Line cookie sheet trays with parchment paper or a silicone baking mat.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and both sugars. Cream until light and fluffy.
Add the eggs and vanilla. Mix well and scrape the bowl with a spatula.
In a separate bowl, stir together the flour, salt, and baking soda. Add dry ingredients to the butter mixture until just combined, about 1 minute.
Add the chocolate chips and ½ cup of the M&M's. Mix gently on low speed until just combined, about 30 seconds.
Scoop cookie dough balls (about 2 Tbsp each) and place onto prepared baking sheet about 3-inches apart. (8 cookies per tray.) Generously press M&M's into the tops of cookie dough balls (about 6-7 M&M's per cookie) maintaining a round shape.
Bake at 350˚F for 8-10 minutes until edges are just barely starting to brown. For thicker cookies, press edges inward with a cookie spatula while still warm. Allow to cool for 1-2 minutes on the tray, then transfer to a wire rack to cool completely.
