Using a double boiler, or a heatproof bowl over a pot of simmering water (do not to let the bottom of the bowl touch the water) over medium-high heat, add the chopped chocolate, butter, and sugar. Note: To avoid the chocolate from seizing and turning into a stiff, thick mass, do not stir the mixture. It should take about 10 minutes for the chocolate to melt. When the chocolate has melted, remove the bowl from the pot of hot water. Add the egg yolks, vanilla extract, and optional chocolate liqueur or coffee/expresso to the chocolate mixture and stir together until well blended. The texture of the mixture will be dense and paste-like. Set aside.
In a separate medium-sized bowl, add the egg whites and whip on high speed with an electric mixer until stiff, soft peaks form. Using a rubber spatula gently fold 1/3 of the egg whites into the chocolate mixture until completely blended. Carefully add the remaining egg whites by gently folding them into the chocolate mixture. Be certain to blend all of the egg whites into the chocolate mixture until a pudding consistency.
Pour the chocolate mixture into the serving bowl of your choice. Cover tightly with plastic wrap. Refrigerate for 12 hours until firm and chilled. (This recipe is even better when prepared a day in advance.)
One hour before serving prepare whipped crème (see recipes below). Place on top of chocolate mixture. Using a carrot scraper, add shaved chocolate to decorate the top.
In a chilled, medium-sized bowl, beat whipping cream, sugar and vanilla with an electric mixer on high speed until stiff peaks form.
Variation - Make Spiced Whipped Creme by adding 1 teaspoon each of orange extract and orange zest and 1/2 teaspoon of ground cinnamon
Dark Chocolate Mousse Recipe https://whatscookingamerica.net/elleneaston/darkchocolatemousse.htm