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

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars together on medium speed until light and fluffy, about 3 minutes. Scrape the bowl down before adding in the vanilla extract and egg, beating well to combine for 1 minute.

In a separate bowl, whisk together the flour, cocoa, cornstarch, salt, and baking soda. Gradually add the dry ingredients to the wet ingredients and beat until a slightly sticky dough forms. Fold in white chocolate chips and M&Ms with a rubber spatula.

Using a large spring-loaded cookie scoop, drop 3-tablespoon sized balls of dough onto the prepared baking sheets, spacing them at least 2 inches apart. Dot the balls of cookie dough with extra chips, M&Ms, and candy eyeballs. Bake for about 12 minutes, until puffy but still soft, being careful not to overbake. Allow to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.