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 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 chocolate 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 until stiff peaks form. 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 of cream until stiff peaks form. 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.
Eggs are easier to separate when cold. Egg whites whip up easier when closer to room temperature. I recommend separating them in advance if you have the time.
When whipping the egg whites, make sure there’s no water or liquid in the mixer as it’ll prevent the egg whites from forming peaks.
Please make sure the chocolate mixture has cooled before adding it to the beaten egg yolks. If the mixture is still hot, you risk scrambling the egg yolks.
While the chocolate mixture shouldn’t be too hot, it shouldn’t be allowed to cool for too long. You don’t want the chocolate to harden and set before mixing with the eggs. Doing so will lead to a lumpy chocolate mousse.
If chopped chocolate doesn’t melt fully in the heavy cream, you can return the saucepan to low heat and stir to melt.
To make chocolate shavings for the topping, you can scrape the chocolate bar with a knife on an angle before chopping it to melt.