Beat 1/2 cup cream in a medium bowluntil stiff peaks form; cover and chill.
Combine egg yolks, espresso, salt,and 2 tablespoons sugar in a large metalbowl. Set over a saucepan of gentlysimmering water (do not allow bowlto touch water). Cook, whiskingconstantly, until mixture is lighter incolor and almost doubled in volumeand an instant-read thermometerinserted into the mixture registers160°F, about 1 minute.
Remove bowl from pan.
Addchocolate; whisk until meltedand smooth.
Let stand, whiskingoccasionally, until room temperature.
Using an electric mixer, beat eggwhites in another medium bowl onmedium speed until foamy. Withmixer running, gradually beat inremaining 1 tablespoon sugar. Increasespeed to high and beat until firmpeaks form.
Fold egg whites into chocolate in2 additions; fold whipped cream intomixture just to blend.
Divide mousse among six teacupsor 4-ounce ramekins. Chill until firm,at least 2 hours. DO AHEAD: Moussecan be made 1 day ahead; coverand keep chilled.
Let stand at roomtemperature for 10 minutes beforeserving.
Before serving, whisk remaining1/4 cup cream in a small bowl until softpeaks form; dollop over mousse.
