Step 1: Melt the Chocolate

1. In a heatproof bowl, combine the chopped dark chocolate, brewed espresso, and butter.
2. Set the bowl over a pot of simmering water (double boiler method) and stir until melted and smooth.
3. Remove from heat and let cool slightly.

Step 2: Prepare the Egg Mixture

1. In a separate bowl, whisk together the egg yolks, granulated sugar, and vanilla extract until the mixture is pale and slightly thickened.
2. Gradually add the melted chocolate mixture to the egg yolk mixture, stirring until well combined.

Step 3: Whip the Egg Whites

1. In a clean mixing bowl, beat the egg whites with a pinch of salt until soft peaks form.
2. Gently fold the whipped egg whites into the chocolate mixture in three additions, being careful not to deflate it.

Step 4: Whip the Cream

1. In another bowl, whip the heavy cream until soft peaks form.
2. Gently fold the whipped cream into the chocolate mixture until no white streaks remain.

Step 5: Chill the Mousse

1. Spoon or pipe the mousse into individual serving cups or ramekins.
2. Cover and refrigerate for at least 2–3 hours or until set.

Step 6: Serve

1. Just before serving, top each mousse cup with whipped cream, chocolate shavings, and a coffee bean if desired.
2. Serve chilled and enjoy the rich, creamy flavors!