Big Soft Batch M & M Peanut Butter Cookies

brown sugar, firmly packed

white sugar

butter, softened

creamy peanut butter

large eggs

vanilla extract

salt

+ ¾ cups all-purpose flour (use the fluff, spoon, sweep method )

+ ½ teaspoons baking soda

+ ½ teaspoons cornstarch

plain M & M's, divided