Chop the dark chocolate into small pieces and melt it using a double boiler or microwave, then let it cool slightly.

Whip the heavy cream in a separate bowl until it forms soft peaks, adding vanilla extract if desired.

In another bowl, beat egg yolks with granulated sugar until light and creamy, then fold in the cooled melted chocolate.

Gently fold half of the whipped cream into the chocolate mixture.

In a clean bowl, beat the egg whites until stiff peaks form, then fold into the chocolate mixture in two additions.

Transfer mousse to molds or cups, smooth the tops, cover, and chill for at least 2 hours.