Adjust oven rack to middle position and heat oven to 425 degrees. Line 2 baking sheets with parchment paper.
Combine flour, salt, and baking soda in bowl. Whisk to Combine.
2 ¼ Cup All Purpose Flour, 3/4 tsp Baking Soda, 1 tsp Table Salt
In another large bowl, Whisk melted butter, brown sugar, and granulated sugar until thoroughly combined, about 30 seconds.
12 Tbsp Unsalted Butter Melted, 1 Cup Packed Light brown sugar, 1/2 Cup Granulated Sugar
Whisk in egg and yolk and vanilla until fully combined and mixture looks emulsified, about 30 seconds.
1 Large Egg plus 1 Large Egg Yolk, 2 tsp Vanilla
Stir in half of flour mixture with rubber spatula or wooden spoon. Stir in M&M's and remainder of flour mixture . Mix until just combined.
1 1/4 Cup M&M candies (reserve some for tops of cookies-see note)
Scoop cookies in 2 Tablespoons size portions ( you can go bigger or smaller if you want). Roll dough portions between your hands to make smooth balls and place evenly on prepared cookie sheets, 8 per sheet. Gently press each ball with your hand- slightly to about 3/4 inch. Press extra M&M's into tops of each cookie.
Bake cookies, 1 sheet at a time, until centers of cookies are puffed and still very light without browning, about 6-8 minutes; they may still seem underdone. Let cookies cool on sheet for 5 minutes. Using spatula, transfer cookies to wire rack and let cool.
These store well for several days in an airtight container and they freeze great too!