1Spray 13x9-inch pan with cooking spray. Crush 3 cups of the cereal; pour crushed cereal into medium bowl. Add 1/2 cup of the melted butter; mix thoroughly. Press mixture in bottom of pan. Refrigerate 1 hour.
2Remove ice cream from container in one block. Cut ice cream lengthwise into 4 pieces; place on top of cold crust. Sprinkle remaining 1/2 cup cereal over top. Place in freezer.
3In 2-quart saucepan, mix powdered sugar, evaporated milk, chocolate chips and remaining 1/2 cup butter. Heat to boiling. Boil 8 minutes, stirring constantly. Cool about 10 minutes or until warm.
4Remove pan from freezer; pour warm chocolate sauce over top. Return to freezer; freeze at least 1 hour longer or until set before serving. To serve, cut into 6 rows by 3 rows. Store in freezer.