
Preheat oven to 350 degrees F. 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.
In a separate bowl, whisk together the flour, cocoa, cornstarch, salt, baking soda, and baking powder. Gradually add in the dry ingredients to the wet ingredients and beat until a slightly sticky dough forms. Fold in the 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.
Bake for about 12 minutes, until puffy but still soft, being careful not to overbake. Carefully press the candy eyeballs into the cookies while still hot from the oven. Allow to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.