Cream together the butter and sugars in a mixer. Add the eggs and vanilla, making sure to scrape down the sides of the mixing bowl. Add the flour, salt, baking soda and baking powder and mix on low until everything is incorporated. Fold in 1/2 of the bag of M&M's.
Chill the dough in the refrigerate for 2 hours. Scoop out some dough, and roll it into a ball. It should be about the size of a golf ball.
Roll the ball of dough around in the additional M&M's, making sure the M&M's stick into the dough, and place the cookie on a parchment lined baking sheet. Place 6 cookies on a sheet since they will spread due to their size.
Bake for 14-15 minutes until just slightly golden around the edges. Remove from oven and let rest on the baking sheet. If you want to flatten them out a big, carefully bang the baking sheet on the counter to release any excess air in the cookies. Note The might look a little under baked in the middle - but don't fear!! They will continue to bake a bit once removed from the oven.
Also - let's talk about how we measure flour. For this recipe, and all of my baking recipes, I scoop the flour and then level it out! That's what works best for me so do it that way when you make these cookies!