Set a medium saucepan filled halfway with water over medium low heat.
In a large bowl, beat the egg yolks and 3/4 cup of sugar with a whisk until the sugar is incorporated. It should be thick and pale yellow in color.
Beat in the liqueur, if using.
Place the bowl over the pot of the almost simmering water (don't boil!) and beat the egg mixture for another 3-4 minutes until foamy and hot. T
ake the bowl from the stove and beat the egg mix over cold water for another 3-4 minutes until cooled.
The mix should be able to make a ribbon if lifted from the bowl and have a mayonnaise texture.
In a separate medium sized bowl (glass or metal), melt the chocolate and coffee together over the pot of hot water. This is a double boiler. Once melted, remove from the heat and whisk in the softened butter, one small piece at a time until melted and smooth.
Slowly beat the chocolate mixture into the egg yolk and sugar mix. You can also add in some of the glazed orange peel if wanted.
In a large bowl attached to a mixer, beat the egg whites with a pinch of salt until softly peaked. Sprinkle in 1 tablespoon sugar and beat until stiff peaks are achieved.
Stir 1/4 of the egg whites into the the chocolate mix, then fold in the rest.
Place the mousse into individual cups, dishes, or ramekins.
Chill for at least 2 hours.
Top with desired additions.