PREHEAT oven according to package directions. Grease 13 x 9-inch baking pan.
PREPARE brownie mix according to package directions, using vegetable oil and egg(s) and substituting evaporated milk for water. Spread into prepared baking pan.
BAKE according to package directions; do not over bake. Remove from oven. Immediately sprinkle with chocolate morsels. Let stand 5 minutes or until morsels are shiny. Spread evenly. Top with marshmallows and walnuts.
BAKE for 3 to 5 minutes or just until marshmallows begin to melt. Cool in pan on wire rack for 20 to 30 minutes. Cut into squares using a knife dipped in hot water.