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

In a stand mixer, cream together the unsalted butter, granulated sugar, and brown sugar until light and fluffy.

Add vanilla extract and egg, mixing until combined.

In a separate bowl, whisk together flour, cocoa powder, cornstarch, salt, baking soda, and baking powder.

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

Fold in white chocolate chips and Halloween M&Ms.

Scoop 3-tablespoon balls of dough onto prepared baking sheets, spacing them out.

Bake for 12 minutes, keeping an eye on them until puffed but still soft.

Let the cookies rest on the baking sheet for a few minutes before transferring to a wire rack to cool completely.