Place the finely chopped chocolate, espresso powder, and the egg in an high powder blender. In a sauce pan heat 1/4 cup of water until boiling. Pour the boiling water into the chocolate and blend on high speed until smooth.
Heat 2/3 cup of the heavy cream until very warm, but NOT boiling (you can do this in the same pan you boiled the water!). Stream the cream into the chocolate mixture as you blend. Add the vanilla extract and salt, and blend on high speed for 1 minute. The mixture should be creamy and smooth with no pieces of chocolate remaining.
In a separate bowl whisk the last 1/2 cup of cold heavy whipping cream until soft peaks form. Pour the chocolate mixture into the whipped cream and gently fold until completely combined and no pockets of unmixed whipped cream remains.
Spoon the mixture into 4 to 6 individual containers depending on how big you want your servings to be. Refrigerate for at least 3 hours to set before serving with fresh whipped cream, berries, and chocolate shavings!
Notes
Make sure you chocolate is chopped finely to ensure no little pieces interrupt the smooth texture of your mousse!