Heat oven to 350 degrees F. Line 13 x 9 x 2-inch pan with nonstick foil. Line bottom of pan with graham crackers, breaking apart to fit. Prepare brownie mix as package directs for cakey brownies, adding chocolate chips and walnuts. Spread batter over grahams. Bake in 350 degree F oven for 28 minutes. Top with marshmallows; return to oven for 2 minutes, then cool. Cut into squares with a lightly greased knife.