In a large saucepan, combine the sugar and water and spread evenly over the bottom of the saucepan. Heat the sugar mixture over medium heat stirring until the sugar has melted. Once the sugar is melted stop stirring and allow the sugar to continue to cook. Swirl the pan occasionally to keep the caramel cooking evenly. Allow the mixture to come to a deep amber colour (it will begin to smoke) and remove from the heat immediately. Whisk in the butter until melted then whisk in the heavy cream in a slow and steady stream. Be careful as the mixture will bubble vigorously.

Add in the chocolate and stir gently until the mixture is melted and smooth. Scrape the mixture into a large bowl and let it cool to room temperature. Once it has reached room temperature, whisk in the egg yolks.

In the bowl of a stand mixture fitted with the whisk attachment, whisk the egg whites on high speed until stiff. Fold on third of the egg whites and the salt into the chocolate mixture to lighten it. Fold in the remaining egg whites into the chocolate just until no streaks of white remain. Divide the mousse into the serving glasses or bowls. Chill for at least 8 hours.