1. Heat oven to 350°F. Grease 13x9x2-inch baking pan.
2. Place butter and 1 cup mint chocolate chips in large microwave-safe bowl. Microwave at MEDIUM (50%) 1 to 1-1/2 minutes or until chips are melted when stirred. Add sugar, flour, cocoa, vanilla, baking powder, salt and eggs; stir with spoon until smooth. Stir in remaining 2/3 cup chips. Spread batter in prepared pan; sprinkle walnuts over top.
3. Bake 30 minutes or until center is set. Cool completely in pan on wire rack. Cut into bars. Makes 24 to 36 brownies.