Place mix in small deep bowl (about 1 1/2 quart size). Add boiling hot milk; mix well. Beat vigorously with rotary beater or high speed electric mixer until filling stands in peaks (about 3 to 6 minutes). Add sugar slowly, while beating, beat a minute longer. Fold in marshmallows and pecans. Spoon into pie shell. Chill until set, about 2 hours. Top with whipped cream, if desired.