Line 15x10x1-inch pan with foil; lightly grease foil. Set aside. Melt butter in large saucepan on low heat. Add marshmallows; cook until melted, stirring frequently. Remove from heat. Add cereal; mix well. Press firmly into prepared pan; cool completely.
Remove formed cereal mixture from pan by turning pan upside down on cutting board. Remove pan; peel off foil. Cut cereal mixture crosswise in half. Spread one half with dulce de leche; top with remaining half to form sandwich.
Decorate with icing, if desired. Cut into 24 squares to serve.