Preheat oven to 350°F. Grease 9-inch square baking sheet.

Combine sugat, butter, and water in large microwaveable bowl. Microwave on HIGH 2 1/2 to 3 minutes or until butter is melted. Stir in semisweet chips; stir gently until chips are melted and the mixture is well blended. Stir in vanilla; let stand 5 minutes to cool.

Beat eggs into chocolate mixture, 1 at a time. Combine flour, baking soda, and salt in small bowl; add to chocolate mixture. Stir in mint chocolate chips. Spread into prepared pan.

Bake 25 minutes for fudgy brownies or 30 minutes for cakelike brownies. Remove pan to wire rack; cool completely. Cut into 2 1/4-inch squares. Sprinkle with powdered sugar, if desired.