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. (You may substitute crabmeat or shrimp for the lobster.) A Craig Claiborne recipe.