In a large mixing bowl cream both sugars, the butter and peanut butter together on medium speed until light and fluffy 3 - 4 minutes.
Beat in the eggs, vanilla and salt until well combined.
In a separate bowl whisk together the flour, baking soda and corn starch. Beat in half at a time on low speed just until combined.
Stir in 1 + 1/2 cups of M & M's by hand.
Cover the bowl with plastic wrap and refrigerate the dough for 1 hour or up to 24 hours.
Preheat oven to 350 degrees F. Line 2 large aluminum baking sheets with silpats or parchment paper. If the dough has been refrigerated for more than an hour or two it may need to sit out for a few minutes so it's easier to work with.
Scoop the dough up with a 1/4 measuring cup and roll into balls. Place 3 inches apart on the baking sheets.
Bake 12 minutes or until the bottoms are very lightly golden brown. Remove from oven and press a few m & m's into the top of each one. Cool on pan 5 minutes then remove to wire racks to cool completely.
Store tightly covered up to 1 week.