Put the chopped chocolate
in a medium heatproof bowl
and set the bowl in a skillet
of barely simmering water. Stir the chocolate with a heatproof spatula just until it is melted. Remove the bowl from the skillet, add the butter to the chocolate, and stir until the butter is completely melted and the mixture is smooth.
In a medium bowl with an electric hand mixer on medium-high speed (or with a balloon whisk), whip the egg whites and salt until they barely hold soft peaks. While whipping, gradually sprinkle in the sugar-go slowly, as adding it too fast may cause the whites to fall. Continue whipping until the whites just start to hold stiff peaks. Don't overbeat or the dissolved sugar may weep out of the whites.
Wipe the beaters (or whisk) clean and then whip the cream in a large bowl until it's fairly thick and holds a soft peak when the beaters are lifted.
With a large spatula, gently fold about onethird of the egg whites into the chocolate until the mixture is no longer streaky. Fold in the remaining whites. Scrape the chocolate mixture into the whipped cream. Add a flavoring, if using (see variatons below). Fold gently until the mixture is uniform in color and texture.
Divide among 4 dessert dishes and serve immediately, or refrigerate for at least 30 minutes for a slightly firmer texture. Garnish with chocolate shavings, if using.
The risk of salmonella infection from consuming raw egg whites is low, but the only way to be completely safe is to use pasteurized egg whites.