PREHEAT oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan. Grease foil. Microwave unsweetened chocolate and 3/4 cup of the butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Blend in eggs. Add flour; mix well. Spread into prepared pan.
BAKE 30 to 35 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan.
STIR peppermint extract into frosting; spread evenly over cooled brownies. Microwave semi-sweet chocolate, remaining 1/4 cup butter and the water in medium microwaveable bowl on HIGH 1-1/2 min. or until butter is melted. Stir until chocolate is completely melted. Cool to room temperature. Spread evenly over frosting; cover. Refrigerate at least 1 hour before serving. Lift out of pan onto cutting board, using foil handles. Cut into 36 squares.