1Heat oven to 350°F. Spray bottom only of 8-inch square pan with cooking spray. Place cereal in resealable food-storage plastic bag; seal bag and crush with rolling pin to make about 1 1/2 cups.
2In medium bowl, mix cereal, brown sugar and baking soda; stir in 1/4 cup melted butter until well mixed. Reserve 1/3 cup cereal mixture for topping. Press remaining cereal mixture evenly in bottom of pan. Bake 5 minutes. Cool 5 minutes.
3Meanwhile, in medium bowl, stir brownie mix, 1/4 cup melted butter and the eggs until well blended. Drop batter by small spoonfuls over baked layer. Carefully spread over baked layer; sprinkle with reserved 1/3 cup cereal mixture.
4Bake 30 to 34 minutes or until brownies look dry and set. (Brownies will be soft; do not use toothpick test). Cool completely, about 1 1/2 hours. For brownies, cut into 4 rows by 4 rows. Store tightly covered.