PREHEAT oven to 350° F. Grease a 13 x 9-inch baking pan.
PLACE whole bar of dough in prepared pan. Allow to soften for 5 to 10 minutes. Using fingertips, pat dough gently to cover bottom.
BAKE for 11 to 13 minutes or until edges are golden brown. Sprinkle marshmallows over cookie; bake for an additional 1 to 2 minutes, or until marshmallows are puffed. Distribute pretzels, sprinkles and morsels over marshmallows; press down lightly. Cool completely in pan on wire rack. Cut into bars with wet knife.