Microwave chocolate squares and butter in a large microwave-safe bowl at HIGH 1 1/2 minutes, stirring after 1 minute and then every 30 seconds or until melted. Stir until smooth. Stir in sugar. Add eggs, 1 at a time, beating with a spoon just until blended after each addition.
Stir in flour and vanilla; stir in chocolate morsels. Pour mixture into a lightly greased 13- x 9-inch baking pan.
Bake at 350° for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack.
Spread Cappuccino Buttercream Frosting evenly over top of cooled brownies. Garnish, if desired. Cut into squares. Cover and chill, if desired.