spray 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 into bottom of pan. refrigerate 1 hour.
remove 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.
in 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.
remove pan from freezer, pour warm chocolate sauce over top. return to freezer, freeze at least 1 hour longer or wait until set before serving. to serve, cut into 6 rows by 3 rows. store in freezer.