In a stand mixer, cream together 1 cup of softened unsalted butter with ¾ cup light brown sugar and ½ cup granulated sugar for about 2 minutes until the mixture is light and fluffy. Add in 1 large egg and 1 teaspoon of vanilla extract, mixing until well combined and smooth.

In a separate bowl, whisk together 2 ¼ cups of all-purpose flour, 1 teaspoon of cornstarch, 1 teaspoon of baking soda, and ½ teaspoon of salt. Gradually add this dry mixture into your wet ingredients in the stand mixer, mixing on low speed until just combined.

Gently fold in 1 ½ cups of red, white, and blue M&M's into the dough using a spatula.

Scoop out portions of the cookie dough onto a baking sheet lined with parchment paper, spacing them about 2 inches apart. Top each dough ball with a few extra M&M's for a decorative touch. Cover and chill the dough in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C). Once the dough has chilled, bake the cookies for 10-11 minutes.

After baking, allow the cookies to cool on the baking sheet for about 5 minutes. Then, carefully transfer them to a wire rack to cool completely.