Whip the cream to soft peaks, then refrigerate.

Combine the 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 cool until the chocolate is just slightly warmer than body temperature. To test, dab some chocolate on your bottom lip. It should feel warm. If it is too cool, the mixture will seize when the other ingredients are added.

Once the melted chocolate has cooled slightly, whip the egg whites in a medium bowl until they are foamy and beginning to hold a shape. Sprinkle in the sugar and beat until soft peaks form.

When the chocolate has reached the proper temperature, stir in the yolks. Gently stir in about one-third of the whipped cream. Fold in half the whites just until incorporated, then fold in the remaining whites, and finally the remaining whipped cream.

Spoon or pipe the mousse into a serving bowl or individual dishes. If you wish, layer in fresh raspberries and whipped cream. Refrigerate for at least 8 hours. (The mousse can be refrigerated for up to a day.)

Optional. Garnish with raspberries and extra whipped cream

Serves 5-8, depending on the size of the servings