Preheat oven to 350 degrees. Spray a aluminum foil lined 9" x 13" pan with cooking spray.
Microwave chocolate and butter in a large mixing bowl for 2 minutes. Stir until chocolate is completely melted.
Add sugar and mix well. Beat in eggs and vanilla. Add flour and mix well.
Bake for 27-30 minutes or until toothpick inserted comes out with fudgy crumbs.