Sift the flour, cornstarch, baking soda, and salt in a separate bowl. Then set it aside.
Mix the butter, sugar, eggs, and vanilla
Now melt the unsalted butter in a saucepan or microwave and pour it into a bowl. Make sure the butter isn’t too hot. Melt it just until most of the butter has melted. It’s ok if there is still a little chunk of softened butter.
Add the brown sugar and granulated sugar to the butter along with the vanilla extract. Mix that together using an electric hand mixer. Make sure to beat it well.
Now add the room temperature egg and egg yolk and beat them in. Room temperature eggs are important here. If you forget to leave them out, just place the eggs in a bowl of hot water for 5-10 minutes and they should be the perfect temperature.
Combine the wet and dry ingredients + m&ms
Next, add in the dry ingredients and gently mix on low speed. Then gradually pick up the speed and mix until all ingredients are fully incorporated. Lastly, mix in the m&m candies.
Now scoop out mounds using a cookie scoop and roll the cookie dough into your hands. Place the cookies on a baking sheet lined with parchment paper about 2 inches apart. You can also add a few extra m&ms on top for a more colorful cookie.
Chill and bake (chilling is optional)
Cover and chill your dough for about 2 hours. This step is optional. If you want flatter cookies, don’t chill the dough.
Then bake at 350 degrees Fahrenheit for about 12-13 minutes. The centers will look a little under-baked but don’t worry, they will continue to bake even after you take them out of the oven.
Let them sit on the baking sheet for about 5 minutes before transferring them to a cooling rack. Keep them in an airtight container and enjoy for up to 1 week.
How far in advance can I make them?If you bake them right away, they will stay fresh for up to 2 days in an airtight container. If you freeze the dough, it will keep for 6 to 12 months. How to freeze M&M cookie doughTo freeze the cookie dough, just place it in an airtight container once it has been shaped and chilled. You can use a container or a ziplock freezer bag to store the rolled dough. In the freezer, your M&M cookies will last for 6-12 months.