Preheat oven to 350 degrees F. Line baking sheet with parchment.
Beat butter and sugars till creamy.
Add egg and vanilla.
Add flour, cornstarch, baking soda and salt.
Fold in M & M's
Using a cookie scoop drop dough onto sheet.
Bake about 8 minutes.
Remove from oven and press one chocolate piece into each cookie with 3 marshmallows.
Return to the oven for 4 more minutes or until baked through and marshmallows have poofed!
Let cool on cookie sheets.