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

In a stand mixer, combine the unsalted butter with the granulated and brown sugars. Beat until light and fluffy, about 3 minutes.

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

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

Gradually add the dry ingredients to the wet ingredients, mixing on low until just combined.

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

Scoop 3-tablespoon-sized balls of dough onto the prepared baking sheets, leaving 2 inches apart.

Bake for about 12 minutes, until puffy and set but still soft to the touch.

Let the cookies cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.