PREHEAT oven to 350° F. Grease 8-inch-square baking pan.
PLACE whole brownie bar in prepared pan. Allow to soften for 5 to 10 minutes. Using fingertips, pat dough gently to fill pan.
BAKE for 24 to 26 minutes or until wooden pick inserted in center comes out slightly sticky.
SPRINKLE 1/4 cup morsels over hot brownie. Let stand for 5 minutes or until morsels are shiny; spread evenly over brownie. Top with marshmallows, remaining morsels and nuts; press down lightly.
BAKE for an additional 5 minutes or just until marshmallows just begin to puff. Cool in pan on wire rack for 20 to 30 minutes. Cut into bars with wet knife. Serve warm.