Steps
1. Melt the dark chocolate gently in a heatproof bowl over simmering water.
2. Stir the instant coffee powder into the melted chocolate until fully dissolved.
3. In a separate bowl, whisk the heavy cream with sugar until soft peaks form.
4. In another clean bowl, whip the egg white with a pinch of salt until it forms stiff peaks.
5. Gently fold the whipped cream into the coffee-chocolate mixture.
6. Carefully fold in the whipped egg white, maintaining the airiness.
7. Spoon the mousse into a serving dish and chill in the refrigerator for at least 1 hour before serving.