Prepare brownie mix according to package directions, using oil and egg(s) except substituting evaporated milk for water. Lightly spray 13 x 9-inch baking pan with no-stick spray. Spread into pan.
Bake according to package directions; do not overbake. Remove from oven. Immediately sprinkle with chocolate morsels. Let stand 5 minutes or until morsels are soft. Gently spread the chocolate.
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. Serve warm.