01 - Combine chopped chocolate and unsalted butter in a heatproof bowl over a saucepan of gently simmering water. Stir continuously until smooth and fully melted. Incorporate brewed coffee or espresso if desired, then remove from heat and allow to cool slightly.
02 - In a large bowl, whisk egg yolks with half of the granulated sugar until the mixture is pale and slightly thickened. Blend in the cooled chocolate mixture until completely combined.
03 - In a clean bowl, beat the egg whites with cream of tartar or lemon juice using a mixer or whisk until soft peaks form. Gradually add remaining sugar and continue beating until stiff, glossy peaks are reached.
04 - Using an electric mixer or whisk, whip chilled heavy cream with vanilla extract and a pinch of salt until soft peaks form.
05 - Carefully fold one-third of the whipped egg whites into the chocolate mixture to lighten. Gently fold in the remaining egg whites until just incorporated, maintaining volume.
06 - Fold the whipped cream into the chocolate mixture, using delicate strokes to avoid deflating the mousse.
07 - Spoon the completed mixture into individual glasses or ramekins. Cover and refrigerate for at least 2 hours, or until set.
08 - Present chilled mousse with optional garnishes such as additional whipped cream, chocolate shavings, or fresh berries.