Making

Heat the sugar in a heavy bottom pot over medium heat. As it begins to liquefy, gently stir it. Continue to cook stirring very gently, until all the sugar is melted. Careful; once the sugar starts to darken, it is in danger of burning. 

When the caramel is a deep clear amber colour remove it from the heat and immediately whisk in the butter, until melted. Slowly whisk in the cream until smooth. If the caramel 'seizes' you can gently heat and stir until smooth.

Whisk in the chocolate, until it's melted and smooth. Transfer the mixture into a large bowl and let it sit until it reaches room temperature. Once it's no longer warm, whisk in the egg yolks. 

In a separate bowl, whip the egg whites until stiff peaks are reached. Stir 1/3 of the egg whites into the chocolate mixture, sprinkling in the flaky salt. Fold in the remaining egg whites just until no streaks of white remain. 

Divide the mousse into 6 servings and chill for at least 4 hours.