The best chocolate mousse must be perfect. It should taste rich, indulgent and not too sweet, with a texture that is both light-and-airy and silky-and-smooth. After many attempts we nailed it and made a dessert that not only exceeded our standards, it's also quite easy to pull off. Close your eyes and take a bite -- you'll be transported to a 4-star French restaurant.
Whip 1 cup of the heavy cream to soft peaks in a medium bowl (see Cook's Note). Set aside.
Add the egg, egg yolks, sugar, espresso powder, vanilla extract, salt and 1 tablespoon water to a medium heatproof bowl and whisk to combine. Set the bowl over a medium saucepan of gently simmering water (do not allow the bowl to touch the water). Whisk constantly until the mixture is pale, hot to the touch and has almost doubled in volume, about 4 to 6 minutes, scraping down the sides of the bowl occasionally with a rubber spatula, if necessary. Remove from the heat and continue whisking until cooled, about 2 minutes.
Put the chopped chocolate in a medium heatproof bowl and set over the saucepan of gently simmering water (do not allow the bowl to touch the water). Stir occasionally with a rubber spatula until the chocolate is melted and smooth. Remove from the heat and gently stir for about 3 minutes to cool slightly.
Whisk the egg mixture into the melted chocolate in 3 additions until combined. (The mixture may get very thick.) Using a rubber spatula, gently fold the whipped cream into the chocolate mixture in 3 additions until it is fully incorporated. It is important that the chocolate mixture is not warm to ensure that it combines smoothly with the whipped cream without seizing up.
Divide the chocolate mousse among four 4-ounce ramekins and chill until firm, about 1 hour.
Whip the remaining 3/4 cup cold heavy cream to stiff peaks. Top each chocolate mousse with whipped cream and serve. (For a softer texture, allow the ramekins to sit at room temperature while you whip the cream.)
We whipped our cream manually with a regular whisk, but you can also use an electric mixer.