Heat oven to 350°F. Spray bottom only of 15x10x1-inch pan with cooking spray. In large bowl, beat brownie mix, oil, water and eggs 50 strokes with spoon. Stir in 1/2 cup pecans. Spread batter in pan.
Bake 16 to 22 minutes or until center is set. DO NOT OVERBAKE. Cool completely on wire rack, about 45 minutes.
Spoon ice cream evenly over brownies; smooth with back of spoon or rubber spatula. Freeze uncovered until firm, about 3 hours.
To serve, drizzle hot fudge topping over dessert with fork using quick strokes. Sprinkle with 1/2 cup pecans. Let stand at room temperature about 5 minutes before cutting. Serve topped with whipped topping.