Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper.
In a large mixing bowl, combine the softened unsalted butter, packed brown sugar, and granulated sugar. Cream the mixture until light and fluffy, about 2-3 minutes.
Add the large egg and vanilla extract to the butter-sugar mixture. Beat until well combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and cornstarch.
Gradually add the dry ingredients to the wet ingredients. Mix on low speed until just combined.
Fold in the M&M's and, if desired, the pink sprinkles.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheets.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.
