1) Put the chocolate in a medium heatproof bowl. Bring a saucepan filled with 2.5cm or so of water to a very slow simmer; set the bowl over, but not touching, the water. Stir the chocolate occasionally until melted and smooth. (Alternatively, put the chocolate in a medium microwave-safe bowl. Melt at 50 percent power in the microwave until soft, about 1 minute. Stir, and continue heat until completely melted, about 1 to 2 minutes more.)
2) Whip the cream in a medium bowl until it holds soft peaks. Take care not to over whip the cream or the mousse will be dense. Cover and refrigerate while you cook the eggs.
3) Whisk the water and espresso powder together in a small bowl.
4) Put the eggs in another bowl that also sets over the water in the saucepan. Beat the egg yolks, pinch of salt, and sugar until foamy and light, about 30 seconds. Set the bowl over the water and whip with an electric mixer or whisk, moving in a circular motion around the bowl, until the eggs get very fluffy and hot to the touch, about 3 minutes. Remove the bowl from the heat and continue beating on high speed until the thick ribbons fall from the beater when lifted out of the bowl, about 5 minutes more. Fold in the espresso.
5) Fold about a quarter of the eggs into the chocolate to lighten it, then fold in the rest of the egg. Finally fold the whipped cream into the chocolate base to make a smooth light mousse.
6) Pour the mousse into 4 serving dishes or wine glasses. Cover and refrigerate until set, about 1 hour. Serve. For a dark, double-rich mousse, use all dark chocolate instead of the milk, and decrease the cream to 180ml.