Preheat oven to 375°F. Line two large cookie sheets with parchment paper. Set aside.
Whisk flour, cornstarch, baking soda and salt together in a medium bowl. Set aside.
Heat 10 tablespoons of the butter in a medium heavy saucepan over medium high heat until melted. Let it come to a boil, stirring constantly for 2-3 minutes or until butter turns a dark golden brown and has a nutty aroma. Remove from heat. Place the remaining 4 tablespoons of butter in a glass measuring cup and microwave on high for one minute. Pour into the browned butter and stir to combine. Pour into a large heatproof bowl.
Add both sugars, eggs and vanilla. With an electric mixer, beat on low just until ingredients are combined. Turn it up to medium high and beat for 2 minutes. Stir in the flour mixture until just combined. Fold in the chocolate chips. Cover and refrigerate for at least 1 hour or overnight. (Do not skip this step.)
Divide chilled dough into 12 portions, each about 3 tablespoons. Roll into balls and place on prepared cookie sheets, about 2 inches apart. Press balls down slightly with the palm of your hand. Press about 10 M&M's into the top of each cookie ball. Bake cookies for 9-11 minutes or until they are light golden brown just around the edges. They will still be soft in the middle. Take the cookie sheets out of the oven and place on a cooling rack. Let cookies cool completely before eating. Store in a airtight container for up to 1 week (if they last that long) or store in the freezer for 1 month.