Melt the dark chocolate using a double boiler or microwave in 30-second intervals, stirring between each. Let it cool slightly.
In a mixing bowl, separate 4 large eggs into whites and yolks. Ensure no yolk remains in the whites.
In a clean, dry bowl, whip the egg whites with a pinch of salt until stiff peaks form.
In a separate bowl, beat the egg yolks with powdered sugar until light and fluffy, then gently fold in the melted chocolate.
Carefully incorporate the whipped egg whites into the chocolate mixture using a spatula, alternating with a portion of the batter to maintain airiness.
If using heavy cream, whip it lightly and fold it into the mousse mixture.
Transfer the mousse into serving cups or a trifle bowl, cover with plastic wrap, and refrigerate for at least 2 hours before serving.