Preheat oven to 350°F (180°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.

Combine the flour, cornstarch, baking soda, baking powder and salt in a medium-sized bowl and set aside.

Cream the butter and sugars together in a large mixer bowl on medium speed until light in color and fluffy, about 2-3 minutes. You should be able to see the change in color happen and know it’s ready.

Add the eggs one at a time and mix until well combined after each addition. Add the vanilla extract and mix until well combined.

Add the dry ingredients and mix until mostly combined. Do not over mix.

Add the M&Ms and use a silicone spatula to fold everything together into a uniform cookie dough. The cookie dough will be thick.

Make cookie dough balls that are 4 tablespoons (68g) in size and place onto the prepared baking sheet 2-3 inches apart. They will spread, but not a ton. Use your fingers or a spatula to shape the cookies and make them round. They won’t shape easily after they come out of the oven.

Bake cookies for 8-10 minutes. The cookies will spread a bit and the centers will look soft. Remove just before the edges begin to turn golden. Don’t over bake.

Remove from the oven and allow to cool on baking sheets for 4-5 minutes before transferring to a wire rack to cool completely.