Preparation - Mix butter, sugar, & evaporated milk in a heavy saucepan. Bring to a full boil over medium heat, stirring constantly. Continue boiling for 5 minutes or until a candy thermometer reaches 235º, stirring constantly. Remove from heat. Stir in chocolate chips until melted. Add marshmallow, nuts & vanilla & mix well. Pour into a greased 9" x 13" pan . Cool at room temperature for 2 to 3 hours, then cut into squares.