Preheat the oven to 350°F (175°C). Line a 9×9 baking dish with parchment paper, leaving an overhang for easy lifting.

In a stand mixer fitted with the paddle or in a large bowl with a hand mixer, cream ½ cup softened butter with ½ cup packed light brown sugar and ¼ cup granulated sugar until light and fluffy, about 2–3 minutes.

Add 1 large egg and 1 teaspoon vanilla extract. Beat until the mixture is smooth and combined.

In a separate bowl, whisk together 1½ cups all-purpose flour, ½ teaspoon baking soda, ¼ teaspoon baking powder, ¼ teaspoon kosher salt, and ¼ teaspoon cornstarch if using.

Gradually add the dry ingredients to the wet ingredients and mix just until combined, stopping as soon as there are no dry streaks.

Fold in ¾ cup green M&M’s and ¾ cup semi-sweet chocolate chips, saving a handful for decorating the top.

Transfer the dough to the prepared pan and press it evenly into the pan. Sprinkle the reserved candies and chips on top and gently press them in.

Bake for 20–25 minutes, until the edges are lightly golden and the center is just set.

Cool completely in the pan on a wire rack before lifting out and slicing into squares.