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

In a stand mixer, cream the unsalted butter, granulated sugar, and brown sugar on medium speed for about 3 minutes.

Add the vanilla extract and room temperature egg, mixing until smooth and creamy.

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 ingredients, beating on low speed until a slightly sticky dough forms.

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

Scoop roughly 3-tablespoon-sized portions of dough onto the prepared baking sheets, spacing them at least 2 inches apart.

Dot the tops of each cookie dough ball with extra white chocolate chips, M&Ms, and candy eyeballs.

Bake in the preheated oven for about 12 minutes until puffy and soft.

Cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.