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

In a stand mixer, combine unsalted butter, granulated sugar, and brown sugar. Beat for 3 minutes until light and fluffy.

Add vanilla extract and egg to the mixture. Mix until fully combined.

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

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

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

Drop 3-tablespoon-sized dough balls onto prepared baking sheets, spacing them 2 inches apart. Top with extra M&Ms and candy eyeballs.

Bake for approximately 12 minutes until puffy but soft in the center. Cool on sheets for 5 minutes before transferring to a wire rack.