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

In a stand mixer, cream together the unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 3 minutes. Add the vanilla extract and egg, mixing until combined.

In another bowl, whisk together the all-purpose flour, black cocoa powder, cornstarch, fine sea salt, baking soda, and baking powder. Gradually combine with the wet mixture until a sticky dough forms.

Gently fold in the white chocolate chips and Halloween M&Ms to the dough using a spatula.

Using a large cookie scoop, place dough balls on prepared baking sheets, spacing them 2 inches apart. Garnish with extra chocolate chips, M&Ms, and edible candy eyeballs.

Bake the cookies for 12 minutes, ensuring they are puffy but still soft in the center. Cool on the baking sheet for 5 minutes before transferring to a wire rack.