Start by chopping the chocolate very finely and placing it in a large heatproof bowl. Melt it gently until silky and smooth, either set over a pan of barely simmering water or in the microwave, heating in short bursts of about 30 seconds and stirring in between so it does not catch.
Add the butter, a few cubes at a time, stirring softly until each piece melts into the chocolate. By now, the mixture should feel just slightly warm, not hot.
Stir in the orange juice and the fresh orange zest, taking your time until everything is fully blended and fragrant.
Separate your eggs with care, placing the yolks into the chocolate mixture and the whites into a clean, dry bowl. Stir the yolks into the chocolate until smooth and glossy, then set it aside.
Add a pinch of salt to the egg whites and begin whisking on medium speed. As they turn pale and airy, reaching soft peaks, start adding the sugar little by little, a spoonful at a time, while continuing to whisk.
Once all the sugar is incorporated, increase the speed and whip until the whites are firm, glossy, and hold their shape beautifully on the whisk.
Take a small portion of these whipped whites, about a quarter, and gently stir it into the chocolate mixture. This first addition is simply to loosen the mixture and make it lighter.
Switch to a spatula and fold in the remaining egg whites in batches, working slowly and delicately so you do not deflate all that lovely air. Stop as soon as the mixture is uniform, with no visible streaks.
Spoon the mousse into serving glasses or cups. Give them a light tap on the counter to settle the mixture evenly, then place them in the refrigerator to chill for at least four hours, though overnight will give the finest texture.