Preheat your oven to 160°C (320°F). Line baking sheets with parchment paper.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cornstarch (if using). Set aside.

In a large bowl, cream together the softened butter, granulated sugar, and light brown sugar using an electric mixer on medium speed until light and fluffy, about 2-3 minutes.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Avoid overmixing.

Scoop tablespoons of dough and roll them into balls. Place them on the prepared baking sheets, leaving some space between each cookie to allow for spreading. If desired, press a few extra M&Ms on top of each dough ball for a colorful finish.

Bake in the preheated oven for 12-14 minutes, or until the edges are just starting to turn golden. The centers should still look slightly underbaked for a chewy texture.

Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.