Melt the chocolate using a double boiler or in the microwave, stirring until smooth. Set aside to cool slightly, just warm, not hot.
Add the egg yolks to the chocolate and whisk vigorously until the mixture is smooth and fully combined. This step is easiest when the chocolate is warm enough to blend easily, but not so hot that it cooks the yolks.
In a separate bowl, beat the egg whites until stiff peaks form.
Lighten the chocolate and egg yolk mixture by stirring in about one-third of the beaten egg whites. This helps loosen the texture. Gently fold in the remaining egg whites in two additions. Use a spatula and a lifting motion, fold from the bottom to the top, so the mousse stays airy and light.
Refrigerate the mousse for at least 3 hours.
Serve chilled, on its own or topped with fresh fruit like strawberries, raspberries, or pears.