Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper.

In a stand mixer, cream the softened butter, granulated sugar, and brown sugar for about 3 minutes until light and fluffy.

Mix in the vanilla extract and room temperature egg until fully combined.

In a separate bowl, whisk together the all-purpose flour, black cocoa powder, cornstarch, fine sea salt, baking soda, and baking powder.

Gradually add the dry ingredients to the wet mixture, beating until a slightly sticky dough forms.

Fold in the white chocolate chips and Halloween M&Ms using a rubber spatula.

Scoop tablespoon-sized balls of dough onto baking sheets, spacing them 2 inches apart.

Dot the tops with additional white chocolate chips, M&Ms, and candy eyeballs.

Bake for approximately 12 minutes until puffy but soft in the center.

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