Remove bones and skin from salmon; dice and marinate in cognac for 2 hours.
Place 2 egg whites in blender; then add diced salmon, the marinade, onion juice and lemon juice, season to taste with salt, freshly ground black pepper and cayenne pepper, and blend at low speed until fish and eggs form a smooth paste. Turn to high speed and blend until very smooth.
Put purée in a bowl surrounded with cracked ice; cool. Whip cream until thick and whisk into fish purée. Beat whites of 4 eggs very stiff but not dry, and gently fold into mixture; correct seasoning; remove bowl from ice and pour mixture into a buttered soufflé dish. Stand in a pan of hot but not boiling water and