Microwave chocolate and butter in a large microwave-safe bowl at HIGH 1 1/2 to 2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in sugar. Add eggs, 1 at a time, whisking just until blended after each addition. Gently stir in flour and next 4 ingredients. Pour mixture into prepared pan.
Cream Cheese Frosting: