To prepare the base, heat the orange juice to lukewarm in a heavy saucepan.
Whisk the egg yolks with 3 ounces (90 grams) of the sugar in a large mixing bowl. Whisk in the flour and warm orange juice, then return the mixture to the saucepan.
Cook over medium-low heat, stirring constantly, until the custard is thick. Do not allow it to boil. Remove from the heat.
Stir in the chocolate until completely melted. Stir in the liqueur. Cover this base mixture with plastic wrap to prevent a skin from forming. Hold for use at room temperature. (Unused base can be kept overnight in the refrigerator; it should be brought to room temperature before mixing with the egg whites.).
To prepare the soufflés, brush 4-fluid-ounce (120-milliliter) sized ramekins with melted butter and dust with granulated sugar.
Preheat the oven to 425°F (220°C). Place a sheet pan in the oven, onto which you will place the soufflés for baking. (This makes it easier to remove the hot soufflé cups from the oven.).
Whip the egg whites to stiff peaks with the remaining 1 ounce (30 grams) of sugar. Fold the whites into the chocolate base and spoon the mixture into the prepared ramekins. The ramekins should be filled to within 1/4 inch (6 millimeters) of the rim.
Smooth the top of each soufflé with a spatula and bake immediately.
The soufflés are done when well risen and golden brown on top and the edges appear dry, approximately 12 minutes. Do not touch a soufflé to test doneness.
Sprinkle the soufflés with powdered sugar if desired and serve immediately.