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

In a separate bowl, whisk the egg yolks and granulated sugar together until pale and creamy. Stir in the melted chocolate mixture and vanilla extract until well combined.

In another clean bowl, beat the egg whites with a pinch of salt until stiff peaks form. Gently fold the egg whites into the chocolate mixture in three additions, being careful not to deflate the mixture.

In a separate bowl, whip the heavy cream until soft peaks form. Gently fold the whipped cream into the chocolate mixture until fully incorporated.

Spoon the mousse into individual serving dishes or glasses. Cover and refrigerate for at least 2 hours, or until set.

Serve chilled, optionally garnished with whipped cream, chocolate shavings, or a sprinkle of cocoa powder.