In a small bowl whisk together the mayonnaise, yogurt, Cognac, ketchup,
lemon juice, and salt and pepper to taste and chill the sauce, covered.
Plunge the lobsters into a large kettle of boiling salted water and boil
them, covered, for 10 minutes.
Transfer the lobsters with tongs to a bowl and let them cool until they
can be handled. Crack the shells, remove the meat, and cut it into ¾"
pieces.
Transfer the lobster meat to a large bowl and chill, covered. The lobster
cocktail may be prepared up to this point 1 day in advance.
Halve the mangoes by cutting just to the sides of each pit and, using a
¾" melonball cutter, scoop the flesh from the mango halves. (There
should be about 2 cups.) To the lobster meat add the mango balls, celery,
whole endives, trimmed and sliced thin crosswise, minced chives, and the sauce
and toss the mixture until it is combined.
Divide the lobster mixture among 12 chilled small glasses and garnish each
serving with 1 of the endive leaves and 2 of the whole chives.
