PREPARE and bake brownie mix in 13x9-inch baking pan as directed on package for cake-like brownies.
REMOVE brownies from oven; immediately sprinkle with marshmallows.
PLACE chocolate chunks and milk in small saucepan; cook on low heat until chocolate is completely melted, stirring frequently. Drizzle over marshmallows; sprinkle with walnuts. Cool completely. Cut into 32 brownies.