Chop or shread the cooked lobster meat and refrigerate until dressing has been prepared. For the dressing, place the yolk in a mixing bowl and add vinegar, mustard, tomato paste, salt and pepper and cayenne. Gradually add the oil, beating vigorously with a wire whisk. Juice the lemon. Beat in the lemon juice, tarragon and cognac. Add lobster and tomatoes to the dressing and fold in with a rubber spatula. This may be done in advance and refrigerated for an hour or so.
Prepare the salad: When ready to serve, stack crisped romaine lettuce leaves and slice them into fine shreads. Place shreads on individual salad plates. Top with lobster dressing and serve immediately.
Note: You may substitute crabmeat or shrimp for the lobster.