In a large heatproof bowl, add chopped chocolate and butter. Melt them in the microwave in 30-second intervals, stirring after each increment. When most of the chocolate has melted, remove it from the microwave. Stir it until the rest of the chocolate has melted. Let the melted chocolate cool down for a minute.

(Alternatively, melt the chocolate on top of a double boiler. Place the bowl of chocolate and butter over a saucepan with an inch of simmering water. Stir until the chocolate melts.)

Separate the egg yolks from the whites. Add the whites to a clean medium bowl.

Add the egg yolks to the chocolate mixture, one at a time. Whisk each egg yolk in well before adding the next.

Stir in the orange zest, orange liqueur or orange juice, and a pinch of salt. Set the chocolate mixture aside.

Using an electric whisk or a stand mixer with a whisk attachment, whip the egg whites on high speed until foamy. Stream in the granulated sugar and keep whipping until stiff peaks form and the sugar has dissolved.

Fold the egg white mixture into the chocolate mixture and gently fold it together with a rubber spatula. It should be all incorporated but don't over-mix it, or you will lose the airiness.

Spoon the mixture into four glasses or dessert cups. Place the mousse in the fridge for 5 hours or overnight. If storing overnight, cover the mousse cups with foil or plastic wrap.

Add on a sprinkle of orange zest and chocolate shavings when serving.