Preheat oven to 380 degrees.
In a mixing bowl, cream together the softened butter, sugar, brown sugar, and vanilla. Add eggs and beat in until the batter starts to thicken. (See video for texture.)
Add in the flour, cornstarch, salt, and baking soda. Mix just until combined.
Gently stir in the m&m's, saving a few to press into the tops of the dough balls before baking.
Scoop into 4 ounce balls (or 12 equal portions), place 4 at a time on a parchment lined baking sheet, and decorate the tops with the reserved m&m's. Depending on the size of your cookie sheet, you may be able to fir 6 per sheet. They spread quite a bit during baking, so start with baking 4 at a time to be on the safe side.
Bake in a preheated 380 degree oven for about 15 minutes, or until golden around the edges and still a little gooey in the centers. Remove from oven, place cookie sheet on a rack, and allow to cool for 15 minutes or so on the sheets before transferring. ENJOY!