line 9-inch square pan with foil, ends extending over sides

place butter in 4-qt. microwavable bowl. microwave on high for 1 minute or until melted

add sugar and milk, mix well. microwave 5 minutes or until mixture begins to boil, stirring after 3 minutes

stir well, microwave another 5 1/2 minutes, stirring after 3 minutes

let stand 2 minutes, then add chocolate, marshmallow creme, and vanilla (and nuts)

spread evenly in prepared pan

cool completely at room temperature for about 4 hours