Prepare egg yolk mixture. Whisk the egg yolks and sugar until the mixture becomes thick, pale yellow, and forms a slowly dissolving ribbon when falling back upon itself. Incorporate the orange liqueur. Place the mixing bowl over gently simmering water and continue to whisk for 3 to 4 minutes until the mixture becomes foamy and too hot to touch. Transfer the bowl over cold water and whisk for another 3 to 4 minutes until the mixture cools and regains the consistency of mayonnaise.

Prepare the chocolate mixture. Melt the chocolate with coffee over hot water, remove from heat, and blend in the butter gradually until it forms a smooth cream.

Combine mixtures. Combine the chocolate mixture with the egg yolks and sugar, and optionally, add the orange peel.

Prepare egg white mixture. Whip the egg whites and salt until soft peaks form. Sprinkle with sugar and whip until stiff peaks are achieved. Gently fold one-fourth of the egg whites into the chocolate mixture, and then fold in the remainder.

Transfer. Transfer the mixture into a serving dish, dessert cups, or petits pots. Refrigerate for a minimum of 2 hours, or overnight.

Serve.