Heat oven to 350°F. Spray square pan, 9x9x2 inches, with cooking spray. Mix flour, sugar, cocoa, baking powder and salt in medium bowl. Stir in margarine, milk and egg. Pour into pan.
Bake 13 to 17 minutes or until toothpick inserted in center comes clean and center is set.
Remove from oven; immediately sprinkle with marshmallows. Bake about 2 minutes longer or until puffed. Remove from oven. Sprinkle with cereal; press in lightly.
Spoon melted chocolate and melted butterscotch into separate small resealable plastic food-storage bags. Seal bags; cut tiny hole in one corner of each bag. Squeeze bags to drizzle chocolate, then butterscotch, over brownies. Cool at least 45 minutes. For bars, cut into 4 rows by 4 rows.