01 - Dissolve the instant espresso powder in hot water, stirring thoroughly. Set aside and allow to cool completely.
02 - Place the chopped chocolate in a heatproof bowl set over a saucepan of barely simmering water. Stir gently until completely smooth and glossy. Remove from heat.
03 - Pour the cooled espresso concentrate into the melted chocolate and stir until uniformly blended. Allow the mixture to cool to lukewarm temperature.
04 - In a clean mixing bowl, beat the egg yolks with half of the sugar until the mixture turns pale and thickens slightly. Add the vanilla extract and blend thoroughly. Gently fold this yolk mixture into the lukewarm chocolate espresso base.
05 - In a separate chilled bowl, whip the heavy cream to medium peaks using an electric mixer or whisk. Set aside in the refrigerator.
06 - In a clean, dry bowl, beat the egg whites with a pinch of salt until soft peaks begin to form. Gradually sprinkle in the remaining sugar while continuing to beat until glossy, stiff peaks hold their shape.
07 - Gently fold the whipped cream into the chocolate mixture using a rubber spatula. Then carefully fold in the beaten egg whites in three additions, preserving as much air as possible, until no white streaks remain.
08 - Spoon or pipe the finished mousse evenly into 6 individual serving cups or glasses, smoothing the tops.
09 - Cover the cups loosely with plastic wrap and refrigerate for at least 2 hours, or until the mousse is fully set and firm.
10 - Just before serving, dollop whipped cream on top of each cup and finish with chocolate shavings, a dusting of cocoa powder, or a few espresso beans as desired.