Melt margarine in a large saucepan over medium heat. Add sweetener; stir well. Add marshmallows; cook, stirring constantly, until marshmallows melt. Remove from heat; stir in cereals.
Press cereal mixture evenly into bottom of a 13- x 9-inch pan coated with cooking spray. Let cool at least 1 hour.
Prepare chocolate frosting mix according to package directions. Spread frosting over cereal mixture. Cover and chill 8 hours or until frosting is slightly firm. Cut into 3- x 2-inch bars.