Line 13x9-inch pan with foil, extending foil over sides of pan; spray foil with cooking spray.
Cook brown sugar, 1/4 cup butter and 3/4 cup milk in heavy 3-quart saucepan over medium-high heat, stirring constantly, until sugar is dissolved. Heat to boiling, stirring constantly. Reduce heat to low; boil gently without stirring 5 minutes. Remove saucepan from heat. Stir in 2 cups marshmallows, the vanilla chips and 1 teaspoon vanilla; stir until marshmallows and chips are melted and mixture is smooth. Stir in chopped walnuts. Quickly spread mixture in pan. Refrigerate 30 minutes before preparing Chocolate Fudge layer.