Start by melting the dark chocolate: Place the chopped chocolate in a heatproof bowl over simmering water (double boiler method) and stir until fully melted. Remove from heat and let it cool slightly.
In a separate bowl, combine the espresso with the melted chocolate, mixing until fully combined.
In another bowl, whisk the egg whites with a pinch of salt using an electric mixer until soft peaks form. Gradually add granulated sugar and continue whisking until they reach stiff peaks.
In a large mixing bowl, beat the heavy cream and vanilla extract until soft peaks form.
Add the egg yolks into the chocolate-espresso mixture, combining thoroughly.
Gently fold the whipped cream into the chocolate mixture, followed by the whipped egg whites. Use a spatula and be careful not to deflate the mixture. Fold until just combined and creamy.
Spoon the mousse into serving cups or bowls, and refrigerate for at least 2 hours to set.
To serve, garnish with dark chocolate shavings and fresh mint leaves for an elegant touch.