Preheat oven to 350°F. Line baking sheets with parchment paper and set aside.
In a large bowl, add melted butter, both sugars, and vanilla and whisk to combine.
Add eggs and whisk well until the mixture is glossy and a bit lighter in color, about 1 minute. Let the mixture stand for about 5 minutes while you prepare the next ingredients.
In a medium bowl, add flour, baking soda, baking powder, cornstarch, and salt and whisk to combine.
Give the sugar mixture a quick whisk and then add the flour mixture. Use a wooden spoon or a spatula to gently stir the flour mixture into the sugar mixture. Stir until just a few streaks of flour remain.
Add M&Ms and chocolate chips and stir just until blended.
Scoop 1 ½ Tablespoon balls of dough onto parchment-lined baking sheets.
Bake the cookies for 10 to 12 minutes. The cookies will still appear pale and puffy when you remove the tray from the oven.
For extra pretty cookies, immediately swirl a heat-safe round cookie cutter or glass around the cookies when they come out of the oven. Top the cookies with extra M&Ms and chocolate chips and a sprinkle of salt. See recipe note below for more information.
Allow them to cool on the tray for 5 minutes before transferring to a wire rack to cool completely.
Notes
Storage: Store cookies in an airtight container at room temperature for up to 4 days. Adding a slice of bread to the container will help to keep the cookies fresh.Freezing: Cookie dough can also be scooped and frozen, sealed in an airtight container, for up to 3 months. Bake the frozen dough for an extra minute or two (no need to defrost). Check out my tips for freezing cookie dough.For picture-perfect cookies (think perfectly round cookies studded with glossy chips), check out these tips for baking pretty cookies. Swirling the just-baked cookies in a round cookie cutter and adding extra M&Ms and chocolate chips to the cookies right after they come out of the oven help to make these cookies extra appealing. You may choose to sprinkle with a bit of salt, too.These cookies should bake up nice and thick. If you find that your cookies are spreading too much in the oven, don't miss these tips for fixing flat cookies.