PREHEAT oven to 375°F. Mix cake mix, butter and egg until well blended. Press firmly onto bottom of greased 13x9-inch baking pan.
BAKE 16 to 18 min. or until lightly browned. (Do not overbake.)
SPRINKLE evenly with marshmallows, jelly beans and peanuts. Bake an additional 4 to 6 min. or until marshmallows begin to melt. Cool completely in pan on wire rack. Cut into 32 bars to serve. Store leftover bars in tightly covered container at room temperature.