Line cookie sheets with silicone liners or spray with cooking spray. Preheat your oven to 350 degrees.
Use an electric mixer to beat the butter and sugars until smooth and creamy, about 2-3 minutes.
Add the egg and vanilla; beat until smooth.
In a small bowl, whisk together the flour, cornstarch, soda, baking powder, and salt. Stir into the wet ingredients.
Fold in about a cup of the M&Ms. (Save the rest for the tops of the cookies.)
Scoop and roll into small balls, about 1/2 ounce each. Place cookie dough balls about 2 inches apart on prepared pans.
Bake at 350° for about 7-9 minutes, or until set in the middle. For crunchier cookies, bake until they are golden brown.
Immediately press additional M&M's on the tops of the cookies.
Let sit on the pan for 2-3 minutes, then transfer to a wire rack to cool completely.