BUTTER: Melt the butter in a very large, microwave-safe bowl. Once melted, refrigerate it for 3-5 minutes or until it's a room temperature. The butter can't be hot when you add in the sugars or it will melt the sugar and make the cookies greasy.
WET INGREDIENTS: Once butter is cooled to room temperature, stir in the light brown sugar and white sugar until smooth. Stir in the egg, vanilla extract and maple syrup. Stir again until smooth.
DRY INGREDIENTS: Add in the cornstarch, baking soda, baking powder, and salt. Again, stir until smooth. Add in the correctly measured (See Note 1) flour and mix until just combined. Don't over-mix. Add in the chocolate chips and M&M's. Gently stir until combined.
CHILL: Tightly cover the bowl and chill for 1 hour.
OVEN: Preheat the oven to 350 degrees F. Line a pan with parchment paper or nonstick liner.
SHAPE BALLS: Roll balls of dough into tall cylinders (see picture #11 above for shape and size). Place 6-8 cookie balls on a sheet, allowing plenty of room to spread. Dough balls should be about 1.7 ounces if you have a food scale (or the batch should make about 16 total cookies).
BAKE: Bake for 8-12 minutes erring on the side of under-baking to keep them soft and chewy. The cookies will firm up a little more out of the oven, so take them out as soon as the edges start to lightly brown and the top is no longer wet looking.
MAKE 'EM PRETTY (OPTIONAL): Press a few extra chocolate chips or M&M's onto the tops of the cookies as soon as you remove from the oven. Allow cookies to cool on the baking tray for 5 minutes before transferring them to a cooling rack.
STORAGE: We like these cookies best on day 1 and 2 of being made. They do last up to 1 week, but they begin to lose texture and flavor quality. To store, place in an airtight container and keep at room temperature. Wait until cookies are completely cooled before adding to the container.