Melt chocolate and butter gently until smooth, then allow it to cool slightly so it blends properly with other ingredients without affecting their texture.
Whisk egg yolks with sugar until pale and thick, then combine with melted chocolate carefully to create a rich base.
Whip cold cream with vanilla until soft peaks form, ensuring it remains smooth and easy to fold into the mixture.
Beat egg whites with sugar until stiff and glossy peaks develop, providing structure and lightness to the mousse.
Fold whipped cream and egg whites into the chocolate mixture gradually, using gentle motions to maintain airiness.
Portion into serving dishes and chill for at least two hours to set the mousse and develop flavor.