Preheat oven to 375 digress. Place grahams in food processor container; cover. Process until coarse crumbs form. Add margarine and peanut butter; cover. Process until well blended.
Place graham mixture in 13x9-inch glass baking dish; press firmly onto bottom of dish to form crust. Sprinkle with chocolate chips; drizzle evenly with condensed milk, spreading gently to cover chips, if necessary. (Small holes will fill in as dessert bakes.)
Bake 27 to 30 min. or until top is bubbly all over. Cool completely on wire rack. Before serving, sprinkle evenly with marshmallows. Broil, 3 to 4 inches from heat, 1 to 2 min. or until marshmallows are puffed and lightly browned. Let stand 1 min. before cutting into 24 bars to serve. Store in tightly covered container at room temperature.