Arrange the pieces of ladyfingers in a 2-quart, 4-inch deep serving bowl. Sprinkle with the Cognac.
Beat the egg yolks and sugar in a mixing bowl until they are pale yellow and form a ribbon. Beat in flour. Then beat in the boiling milk by droplets. Pour into a clean saucepan and, stirring with a wire whip, boil slowly for 2 minutes. Remove from heat and beat in the vanilla, then the butter.
Beat the egg whites and salt until soft peaks are formed; sprinkle on the sugar and beat until stiff peaks are formed. Fold the egg whites and 3 tablespoons of the macaroons into the hot creme patissiere.
Spoon the cream into the serving bowl or dessert cups and chill for 2 to 3 hours, or until serving time. Just before serving, sprinkle with the rest of the macaroons.