01 -
Cover a small baking tray with a sheet of parchment and set it aside.
02 -
In a big bowl, use a mixer to whip the softened butter, brown sugar, and white sugar until they’re light and fluffy. Then mix in the egg and vanilla until it’s all smooth.
03 -
Grab a smaller bowl and stir the flour, cornstarch, baking soda, and salt together.
04 -
Slowly add the dry mix into the butter-sugar blend, stirring just until it’s all combined. Fold in the chocolate chips and M&Ms so they’re spread evenly through the dough.
05 -
Scoop about 2 tablespoons of dough and roll them into balls. To keep the cookies the same size, use a cookie scoop. Lay them out on the prepared baking sheet. If you like, press a couple more M&Ms on top for decoration.
06 -
Wrap the cookie sheet with plastic wrap and let the dough chill in the freezer for at least an hour—or even overnight if you’ve got the time. If you’re freezing overnight, let them sit on the counter for 10 minutes before baking. You can also throw the dough balls in a zip-top bag and bake them later as needed.
07 -
Warm your oven to 350°F (175°C). Arrange the dough balls on a parchment-lined sheet and bake them for about 10-12 minutes. They may look slightly underdone, but that’s just the trick for a gooey center.
08 -
Right after you pull the cookies out of the oven, lightly press on some extra M&Ms or chocolate chips, if desired.
09 -
Cool the cookies on the tray for a few minutes, then transfer to a cooling rack. Store in a sealed container and enjoy within a week.