Rinse lobster tails. In skillet, combine lobster, champagne, scallions and salt.
Heat to boiling. Reduce heat, cover and simmer 15 minutes.
Remove lobster; add cream to sauce in skillet. Boil quickly until reduced to about 1/3 cup.
With a wire whisk, beat in butter (1 tablespoon at a time) until smooth. Keep warm. Remove lobster from shells and slice into medallions. Spoon some sauce over each.