Preheat oven to 350 degrees F. Line a 9×9 square baking dish with parchment paper and set aside. Add the butter and chocolate chips to a small microwaveable bowl. Heat in the microwave for 30-second intervals, stirring after each, until smooth. Set aside.
In a large mixing bowl, combine the sugar, eggs and vanilla. Mix until creamy. In a medium bowl, combine cocoa powder, corn starch, baking soda and salt. Mix well and gradually incorporate into butter-sugar mixture until combined. Stir in the melted chocolate.
Pour in an even layer into the prepared baking dish. Top with extra chocolate chips, if desired. Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center comes out clean. Let cool for 15 minutes and cut into squares.