See below

Break 4 oz of good quality bitter chocolate into squares and put in a
fireproof dish in a low oven. When the chocolate is soft, after a few
minutes, take it from the oven, stir in 4 well-beaten yolks, then 1
oz. of softened butter, then the juice of 1 orange. Use a Seville
orange when in season; its aromatic flavor comes through better than
that of the sweet orange.

Beat the 4 egg whites as for a soufflé and fold them into the
chocolate mixture. Pour into little pots, glasses, or coffee-cups.
This quantity will fill 6. Put in the refrigerator or a cool larder
until ready to serve.

Should you have some orange liqueur such as Grand Marnier or Curaçao,
add a spoonful in place of the same amount of orange juice.