PREHEAT oven to 350°F. Beat butter and sugar until light and fluffy. Add flour, pecans and cracker crumbs; mix well. Remove 1/4 cup of the crumb mixture; set aside. Press remaining crumb mixture onto bottom of greased 9-inch square baking pan. Bake 18 min. or until lightly browned.
MELT caramels with water in heavy saucepan on low heat, stirring frequently until caramels are completely melted and mixture is well blended. Pour over crust. Sprinkle with reserved 1/4 cup crumb mixture, the marshmallows and chopped chocolate.
BAKE an additional 12 to 14 min. or until marshmallows are lightly browned. Cool. Cut into 24 bars to serve.