In a large bowl, whisk the flour, baking soda, cornstarch, and salt together. Set aside. 2 ¼ cups (280 g) all-purpose flour 1 teaspoon baking soda 2 teaspoons cornstarch ½ teaspoon salt
In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until evenly combined. ¾ cup (1 ½ sticks, 170 g) unsalted butter ¾ cup (150 g) packed light brown sugar ½ cup (100 g) granulated sugar
Whisk in the egg and egg yolk until evenly mixed in. Whisk in the vanilla extract until evenly incorporated. 1 large egg + 1 egg yolk 2 teaspoons pure vanilla extract
With a spatula or wooden spoon, stir in the dry ingredients and mix together. The dough will be soft and thick. Fold in the chocolate chips and the m and m candies. It can be difficult to stir in the chips and M&m’s so do the best you can! 1 cup (170 g) semi sweet chocolate chips 1 cup m and m candies
Cover the dough tightly and chill in the refrigerator for at least 2 hours, up to overnight for less spreading.
When ready to bake, take the dough out of the refrigerator and allow it to slightly soften at room temperature for 10 minutes.
Preheat oven to 350°F (180°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.
Using a small cookie scoop or use a measuring spoon and scoop dough into 2 heaping tablespoons portions (about 50 g). You can also bake these into smaller cookies (about 2 Tablespoons - about 30-35 g).
Bake the cookies for 11-13 minutes (12 minutes was perfect for me!) or until the edges are very lightly browned. If you are baking smaller cookies, then bake for 9-11 minutes. The centers will look very soft but the edges set and golden brown. The cookies will continue to set as they cool. When they come out of the oven I like to press a few extra chocolate chips and M&M’s on top for appearance. Cool on the baking sheet for 10 minutes. Then transfer cookies to a wire rack to cool completely.