Preheat oven to 350°F.
Spray 8" square baking pan with nonstick cooking spray.
Combine flour, cornstarch, baking soda, & salt in small bowl; mix well.
Melt butter in large saucepan over low heat. Add brown sugar; cook & stir until sugar is dissolved. Remove from heat; sift in cocoa & stir until combined. Stir in flour mixture until smooth. (Mixture will be thick). Stir in chocolate chips & vanilla. Add eggs; beat until smooth & well blended. Pour into prepared pan.
Bake 25 to 30 minutes or until toothpick inserted into center comes out almost clean.