1. Bring a saucepan filled with 1-inch or so of water to a slow simmer. Put the chocolate in a medium heatproof bowl and 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 to heat until completely melted, about 1 to 2 minutes more.
2. Meanwhile, 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. Whisk 2 teaspoons warm water and espresso powder together in a small bowl, if using.
3. Put the egg and yolks in another heatproof bowl that also sets over the simmering water in the saucepan. Beat the eggs, sugar, and pinch of salt 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 are fluffy, thick, and warm to the touch, 2 to 3 minutes. Remove from the heat and continue beating on high speed until thick ribbons fall from the beater when lifted over the bowl and the mixture has cooled slightly, about 3 minutes more. Fold in the espresso. (The mixture will seem loose at this point - that is ok.)
4. Fold about a quarter of the chocolate into the eggs to lighten it, then fold in the rest of the chocolate. Finally, fold the whipped cream into the chocolate base to make a smooth light mousse. Pour the mousse into 4 serving dishes or wine glasses. Cover and refrigerate until set, about 1 hour. If desired, dust the tops with cocoa powder before serving.
For a dark, double-rich mousse, use all bittersweet chocolate instead of the semisweet, and decrease the cream to 3/4 cup.
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