To begin, preheat your oven to 350 and spray two pans with non-stick spray.
To a large mixing bowl, add your butter, white sugar and brown sugar and beat together with a mixer till creamy. Add your vanilla and egg and briefly blend in.
To the same bowl, add your flour, cornstarch, baking soda and salt. Mix together till you have a smooth, thick cookie dough.
Add your M&M candies to the dough and fold them in.
Using a medium cookie scoop, evenly space cookie dough balls onto your pans. You should have about 18 cookies.
Bake in the oven for 9-11 minutes, or till the edges are just cooked and the center looks a touch underdone.
Let cool, then devour!
Stored in an airtight container, these cookies taste best within 3-4 days. Alternatively, you can freeze the dough or baked cookies for later use.