Preheat oven to 350F. Line two cookie sheets with parchment or Silicone baking liners. Set aside.
In a medium bowl- whisk together the flour, baking soda, and salt.
In a large bowl fitted with a paddle attachment- cream the butter and sugars on high until smooth and pale.
Add the egg, the yolk and then the vanilla- mixing thoroughly.
Mix in the dry ingredients until combined.
Use a cookie scoop to portion dough 2-3 inches apart on cookie sheet (mine yielded 1 1/2 inch sized rounds-they will spread significantly) Gently flatten dough a little bit with your fingers.
Bake for 10-12 minutes. Or until edges are set and slightly golden and set. Right away- press as many M&Ms as you want in the tops of the cookies.
Allow to cool for 5 minutes and then transfer to a wire rack.