Here is the tested recipe. from the published collection of the Vicomete de Mauduit, the fascinating cookery book to which the late Frances Countess of Warwick contributed a foreword of admiration:
To each person allow one yolk of one liqueur glass of marsala or sherry (preferably the first), and one large tablespoonful of sugar. Whisk all together with an egg whisk in a basin standing in a pan of nearly boiling water. This has: to be whisked for about a quarter of an hour, bringing the pan gradually nearer the fire, till, at the end of the quarter of an hour, it finds itself on it. Leave it on three minutes, whisking all the time. Serve hot in crystal glass: with longues de chat or sweet biscuits.
Text read from the printed page and cleaned; old type leaves the occasional error.