1Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 8- or 9-inch square pan with cooking spray (without flour).
2In medium bowl, stir brownie mix, 1/4 cup butter, the eggs and zucchini until well blended (batter will be thick). Spread batter in pan.
3Bake 33 to 38 minutes or until toothpick inserted 2 inches from side of pan comes out almost clean. Cool completely in pan on cooling rack, about 1 hour 30 minutes.
4In medium microwavable bowl, microwave 3 tablespoons butter and the chocolate uncovered on High about 1 minute, stirring once, until melted. Stir in powdered sugar and enough milk to make frosting smooth and spreadable. Frost brownies. Cut into 4 rows by 4 rows.