Melt the butter in a very large, microwave-safe bowl. Once melted, refrigerate it for 5 minutes. It’s important the butter isn’t 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 the light brown and granulated sugars. until smooth. Stir in the egg, vanilla extract, and maple syrup until smooth.
Add cornstarch, baking soda, baking powder, and salt. Stir until smooth. Add the accurately measured (see note 1) flour and mix until just combined. Don’t overmix.
Gently stir in 1 cup of M&M’s and 1/4 cup chocolate chips until combined.
Cover the bowl tightly and chill for 1 hour.
Preheat oven to 350°F. Line a pan with parchment paper or use a nonstick liner.
Roll dough balls into tall, cylindrical balls. Place 6–8 cookie balls on the pan to give the cookies plenty of room to spread. Dough balls should be about 1.7 ounces.
Bake for 8–12 minutes, erring on the side of underbaking, which keeps them soft and chewy (I like mine right at 9 minutes)! Remember, the cookies will continue to bake a little more out of the oven, so take them out as soon as the edges start to lightly brown.
Remove from oven and press the remaining 1/4 cup chocolate chips into the tops of the cookies (ensures even placement of chocolate and also makes them look pretty!). Allow to cool on the sheet pan for 5 minutes before transferring them to a cooling rack.
Notes
Note 1: Spoon the flour into a measuring cup and level the top with the back of a table knife instead of scooping the flour from the bag into a measuring cup. This method assures the most accurate measurement.Storage: I like these cookies best on days 1 and 2. They do last up to 1 week, but they begin to lose texture and flavor. Wait until cookies are completely cooled, then place in an airtight container and keep at room temperature.