Melt dark chocolate, butter, and espresso together over low heat until smooth, then stir in vanilla extract and allow the mixture to cool slightly before combining with other ingredients.
Whisk egg yolks with sugar until pale and creamy, then slowly mix in the melted chocolate mixture to create a smooth chocolate base without scrambling the eggs.
Beat cold heavy cream with powdered sugar until soft peaks form because this helps create the mousse’s light and airy texture.
Whip egg whites with salt and sugar until medium peaks form, making sure the mixture looks glossy and stable for better structure.
Fold whipped cream into the chocolate base gently, then carefully fold in the egg whites until fully combined while keeping as much air in the mixture as possible.
Spoon the mousse into serving glasses and refrigerate for at least 4 hours so the dessert becomes smooth, rich, and properly set before serving.
