Bring some water to a simmer in a medium saucepan, place chocolate in a mixing bowl and set over the water to melt slowly. Be sure the water is just gently simmering, not boiling, and stir the chocolate as it melts to keep it from scorching.
Once melted, whisk in the orange liqueur. This will make the chocolate seem to seize up, but keep whisking gently and then add the butter, one tablespoon at a time. The butter should smooth out the texture. Don't panic about it.
Stir in egg yolks one at a time, fully incorporating each before adding the next.
In a separate mixing bowl, whip the egg whites until they hold soft peaks. I used a hand-held electric mixer with the whisk attachment, but you can also whisk by hand if you're looking for a workout. Gradually add sugar as you mix.
Whisk 1/4 of the egg white mixture into the melted chocolate, then gently fold in the remainder.
In another bowl, whip the cream until it holds soft peaks. Fold into chocolate mixture.
Gently transfer into serving bowls - you don't want to deflate the fluffiness that you've worked so carefully to achieve. Cover and refrigerate for at least two hours before serving. Top with whipped cream if desired.