Beat 1/2 cup cream in medium bowl until stiff peaks form; cover and chill.
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.
remove bowl from pan. Add chocolate; whisk until melted and smooth. Let stand, whisking occasionally, until room temperature.
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.
fold egg whites into chocolate in 2 additions; fold whipped cream into mixture just to blend.
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.
before serving, whisk remaining 1/4 cup cream in a small bowl until soft peaks form; dollop over mousse.