Using a hand mixer, whip the heavy cream to soft peaks, then place into the refrigerator immediately.
Combine the chopped bittersweet chocolate, butter, and espresso in the top of a double boiler over hot, but not simmering, water, stirring frequently until smooth.
Remove from the heat and let the chocolate mixture cool until it is just slightly warmer than body temperature. To test, dab a bit of chocolate on your bottom lip. It should feel warm but not hot. If it is too cool, the mixture will seize when the other ingredients are added.
While the melted chocolate is cooling slightly, whip the egg whites in a medium bowl with the hand mixer until they are foamy and beginning to hold a shape.
Sprinkle in the white sugar and beat until soft peaks form.
When the chocolate has reached the proper temperature, stir in the yolks and mix well until shiny.
Gently stir in about one-third of the whipped cream.
Fold in half of the whites just until incorporated, then fold in the remaining whites, and finally the remaining whipped cream.
Spoon or pipe the mousse into individual dishes or a large serving bowl. Refrigerate for at least 8 hours. (The mousse can be refrigerated for up to a day.)
Serve topped with whipped cream, chocolate shavings, and fresh raspberries. Enjoy!