Place the dark chocolate chips in a heatproof bowl set over simmering water. Stir gently until melted and smooth. Remove from heat and stir in the brewed espresso. Set aside to cool slightly.
In a separate bowl, whisk the egg yolks with granulated sugar and vanilla extract until pale and slightly thickened.
Slowly pour the slightly cooled chocolate-espresso mixture into the egg yolk mixture, stirring continuously to create a smooth base.
Using an electric mixer, beat the egg whites to stiff peaks.
Gently fold the whipped egg whites into the chocolate mixture in three additions, using a rubber spatula to maintain lightness.
Divide the mousse evenly into serving cups or ramekins. Cover and refrigerate for at least 2 hours, or until set.
Before serving, whip the heavy cream with powdered sugar until soft peaks form. Spoon or pipe the whipped cream over the chilled mousse.
Finish with chocolate shavings on top for garnish and serve immediately.