In a large bowl, whisk together egg yolks and sugar for about 1 minute, just until lightened in color and velvety.

In a small saucepan, heat 1¼ cups (360ml) of cream over medium heat just until it's steaming, about 4 minutes. Remove from the heat.

Add the chocolate and stir constantly until fully melted. Slowly whisk the chocolate cream mixture into the egg yolks until fully combined. Set aside to cool, stirring occasionally, until barely warm to the touch, about 30 minutes.

In another large bowl, beat the egg whites on high speed using a hand or stand mixer with the whisk attachment until stiff peaks form, 3 to 5 minutes. Fold a third of the egg white mixture into the chocolate mixture to loosen it. Fold the remaining egg white mixture in just until a few streaks remain.

In a large bowl, beat the remaining 1 cup (240ml) of cream on medium-high speed until stiff peaks form. (You can use the same beaters without cleaning them first.) Fold whipped cream into the chocolate mixture. Fill 6 to 8 small serving glasses with the mousse.

Chill until set, about 4 hours or up to 2 days. (If chilling for more than a few hours before serving, loosely cover the mousse to keep it from forming a skin and drying out.) Serve topped with whipped cream and chocolate shavings, if desired.