Combine chopped chocolate, water, cocoa powder, and espresso powder in a medium heatproof bowl.
In a medium saucepan, boil 1 inch of water and remove pan from heat. Place the bowl with the chocolate mixture on top and stir mixture frequently until melted and smooth.
In a medium bowl, vigorously whisk the egg yolks with the 1 1/2 teaspoon of the sugar and salt for about a minute, until it get lighter in color.
Add the chocolate mixture to the egg mixture and whisk to combine. Set aside to cool.
Using a stand mixer fitted with the whisk attachment, whip the egg whites on medium speed until foamy. Add the remaining sugar, increase speed to medium high, and whip to soft peaks, about 2 minutes.
Whisk one quarter of the egg whites into the chocolate mixture. With a rubber spatula, carefully fold in the rest of the egg whites until most streaks are gone.
Clean the stand mixer bowl and whip the cream, gradually increasing the speed to high. Whip to soft peaks. With a rubber spatula, fold whipped cream into the chocolate mixture until well combined. Spoon mousse into 6-8 individual cups, cover with plastic wrap, and refrigerate for at least 2 hours.