Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper.
In a large bowl using a stand or hand mixer, beat the butter, sugar, and brown sugar on medium speed until light & fluffy, about 3 minutes. Scrape down the bowl, then add the vanilla extract and egg. Beat to combine.
In a small bowl, whisk together the flour, dark cocoa powder, cornstarch, salt, baking soda, and baking powder.
Gradually add the dry ingredients into the large bowl with the wet ingredients. Beat until a slightly sticky dough forms. Beat in chocolate chips.
Add dough to the prepared pan and spread evenly. Top with chocolate candies and candy eyeballs. Bake for about 12 minutes, until soft and puffy. Be careful not to overbake. Allow to cool in pan for 15 minutes before slicing into bars. Enjoy!