HEAT oven to 350ºF.
COVER bottom of 13x9-inch pan sprayed with cooking spray with half the grahams, breaking as necessary to fit. Prepare brownie batter as directed on package; pour over grahams.
BAKE 20 min. Top with chocolate bars. Let stand 2 min. or until softened; spread to cover brownie. Top with marshmallows; bake 1 min. or until marshmallows start to puff.
COVER with remaining grahams; press lightly into marshmallows to secure. Cool.