Melt the butter in a very large, microwave-safe bowl. Once melted, refrigerate it for 3–5 minutes or until at 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.
Once butter is cooled to room temperature, stir in brown sugar and granulated sugar until smooth. Add egg, vanilla extract, and maple syrup. Stir again until smooth.
Add 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 overmix. Add chocolate chips and M&M’s. Gently stir until combined.
Tightly cover the bowl and chill for 1 hour.
Preheat oven to 350°F. Line a sheet pan with parchment paper or silicone baking mat.
Roll dough balls 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 (the batch should make about 16 total cookies).
Bake 8–12 minutes, erring on the side of underbaking 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.
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 sheet pan for 5 minutes before transferring them to a cooling rack.
Video
Notes
Note 1: If you press a measuring cup into a bag of flour, you’ll pack in way too much flour—which will yield cake-y and less flavorful cookies. Instead, spoon flour into a measuring cup and level the top with the back of a table knife. See a visual here. Even better? Weigh your ingredients on a food scale!Storage: I like these cookies best on day 1 and 2. They last up to 1 week, but they begin to lose texture and flavor quality. To store, place completely cooled cookies in an airtight container and keep at room temperature.