Step 1

Beat ½ cup cream in a small bowl to stiff peaks; cover and chill.

Step 2

Combine egg yolks, espresso, salt, and 2 Tbsp. sugar in a heatproof bowl. Set over a saucepan of gently simmering water (do not let bowl 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°, about 1 minute.

Step 3

Remove bowl from heat. Add chocolate and whisk until melted and mixture is smooth. Let sit, whisking occasionally, until room temperature.

Step 4

Using an electric mixer, beat egg whites in a medium bowl on medium speed until foamy. With mixer running, gradually beat in remaining 1 Tbsp. sugar. Increase speed to high and beat until stiff peaks form.

Step 5

Fold egg whites into chocolate mixture in 2 additions; fold reserved whipped cream into mixture just to blend. Divide mousse among 6 teacups or 4-oz. ramekins. Chill until firm, at least 2 hours.

Step 6

Just before serving, whisk remaining 1/4 cup cream in a small bowl to soft peaks; top each cup of mousse with a dollop of cream.

Step 7

Do Ahead: Mousse can be made 1 day ahead. Cover and keep chilled. Let sit at room temperature 10 minutes before serving.