Step 1:

  • Beat 1/2 cup cream in medium bowl until stiff peaks form; cover and chill.
  • Step 2:

  • combine egg yolks, espresso, salt, and 2 tbsp. Sugar in a large metal bowl. Set over a saucepan of gently simmering water (do not allow bowl to touch water). Cook, whisking constantly, until mixture is lighter in color and almost doubled in volume and an instant-read thermometer inserted into the mixture registers 160 degrees, about 1 minute.
  • Step 3:

  • remove bowl from pan. Add chocolate; whisk until melted and smooth. Let stand, whisking occasionally, until room temperature.
  • Step 4:

  • using an electric mixer, beat egg white in another medium bowl on medium speed until foamy. With mixer running, gradually beat in remaining 1 tbsp. Sugar. Increase speed to high and beat until firm peaks form.
  • Step 5:

  • fold egg whites into chocolate in 2 additions; fold whipped cream into mixture just to blend.
  • Step 6:

  • divide mousse among six teacups or 4-oz. Ramekins. Chill until firm, at least 2 hours. Do ahead: mousse can be made 1 day ahead; cover and keep chilled. Let stand at room temperature for 10 minutes before serving.
  • Step 7:

  • before serving, whisk remaining 1/4 cup cream in a small bowl until soft peaks form; dollop over mousse.
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