Preheat oven to 400°F. Spray pans with vegetable pan spray.
Prepare brownie mix with oil, water and egg according to package instructions; pour into prepared pans.
Bake 22-25 minutes or until toothpick inserted into cake comes out almost clean. Cool in pan on wire rack 10 minutes; remove from pans and cool completely.
In small bowl, combine marshmallow cream and 1/4 cup whipped topping; stir until smooth. Gently stir in remaining whipped topping and mini marshmallows; spoon into cake cavities. Assemble cakes by inverting one layer onto second layer.
In small microwavable bowl, heat half and half on HIGH until hot but not boiling, about 30 seconds. Add chocolate chips and stir until smooth. Spoon over cakes and sprinkle with nuts.