Preheat oven to 350°F and line 3 baking sheets with parchment paper.
Using an electric mixer, beat butter and sugars until light and fluffy, about 2-4 minutes
Beat in eggs and vanilla on medium speed until combined
Add flour, salt and baking soda and beat on low speed until combined. Stir in half of the M&M's.
Roll into 2" balls and bake 2" apart for 8-10 minutes, until cookies are lightly browned
Remove from the oven and press remaining M&Ms into the tops of the cookies. Allow to cool before removing from the pan.