In a saucepan over low heat, combine brewed coffee and instant espresso powder until dissolved. Remove from heat and let cool to room temperature.
In a separate bowl, whisk together egg yolks and sugar until pale yellow and creamy. Gradually add cooled coffee mixture to the egg yolk mixture, stirring continuously.
Return the combined mixture to the saucepan and cook over low heat, stirring constantly, until it thickens and coats the back of a spoon (about 8 10 minutes).
Remove from heat, strain through a fine mesh sieve into a clean bowl, then stir in vanilla extract. Allow to cool completely.
In a large mixing bowl, whip heavy cream until soft peaks form. Gently fold the whipped cream into the cooled coffee mixture.
Melt dark chocolate and butter in a double boiler or microwave in 30 second intervals, stirring after each interval until smooth. Let cool slightly.
Spoon melted chocolate into the bottoms of six individual dessert dishes or glasses, creating a thin shell. Place in the freezer to set for at least 15 minutes.
Spoon coffee mousse on top of the chocolate shell, smoothing the surface with a spatula. Freeze until set, about 4 hours or overnight.
To serve, let stand at room temperature for a few minutes before delicately unmolding onto dessert plates.
Chefβs Insight
Choose high-quality dark chocolate to elevate the flavors of this decadent dessert.