In a high-power blender, combine finely chopped semisweet chocolate, espresso powder, and egg. Heat water to a rolling boil, then pour it into the blender. Blend on high for about 30 seconds until smooth and glossy.

In a saucepan over low heat, warm 2/3 cup of heavy cream until it starts to steam. Gradually stream in warm cream while blending, then add vanilla extract, blending for about 1 minute until smooth.

Whisk remaining 1/2 cup of heavy cream in a separate bowl until soft peaks form, taking about 2-3 minutes.

Gently fold the chocolate mixture into the whipped cream, ensuring it remains light and airy.

Spoon mousse into dessert cups, cover with plastic wrap, and refrigerate for at least 3 hours before serving.