Preheat the oven to 350 degrees, and line two baking pans with parchment paper.
With an electric mixer, mix the butter and sugars until light and fluffy, in a large bowl. Then add in the egg and vanilla.
In a medium bowl, combine the flour, cornstarch, baking soda and salt, and mix well.
Add the dry ingredients to the wet ingredients, and mix until just combined. Then add in the M&Ms.
Place Scoops of cookie dough onto the baking pan, and slightly press the tops down. You can add a few M&Ms to the top of the cookies for decoration. Make sure that they are spaced at least 2 inches apart, as they spread.
Bake for 8-10 minutes, or until the edges are golden brown (do not overbake, as the cookies harden).
