Melt chocolate over a double boiler or in short microwave bursts. Stir until smooth.
Heat heavy cream, espresso powder, cocoa, and vanilla in a saucepan until warm but not boiling.
In a separate bowl, whisk egg yolks and sugar until pale.
Slowly pour warm cream into egg mixture, whisking constantly.
Return the mixture to heat and stir until slightly thickened (about 3 minutes).
Remove from heat and whisk in melted chocolate and caramel sauce until smooth.
Let the mixture cool, then fold in whipped cream gently.
Divide into serving glasses and refrigerate for at least 1.5 hours.
While chilling, make caramel drizzle by melting sugar in a pan until golden.
Stir in butter, cream, and espresso, then remove from heat.
Drizzle over chilled mousse before serving.
Grab a spoon and prepare for pure bliss.