Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper, ensuring your cookies don’t stick.
In a large mixing bowl, combine softened unsalted butter, packed brown sugar, and granulated sugar. Use an electric mixer to cream them together until the mixture is light and fluffy, about 2-3 minutes.
Add in one large egg and vanilla extract. Mix until well combined.
In a separate bowl, whisk together all-purpose flour, baking soda, salt, and cornstarch. Gradually add this dry mixture to the wet ingredients, mixing until just combined.
Gently fold in the M&M's and, if desired, the pink sprinkles for a whimsical touch. Make sure they are evenly distributed throughout the dough.
Using a cookie scoop, drop heaping tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are lightly golden but the centers still look soft.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. Enjoy your delicious Soft M&M Cookies!