Step 1Combine the sugar, water and corn syrup in a wide saucepan over medium heat. Bring the mixture to a boil, and cook until the sugar begins to darken to a rich caramel color.
Step 2When the caramel is a deep amber color and starts to smoke, wait a moment for it to smell just slightly burnt, then remove it from the heat and quickly whisk in the butter, stirring until melted. Gradually whisk in the cream and stir until the little bits of caramel are completely melted. (A few can be stubborn, so be patient. You can strain the mixture if they simply refuse to budge.)
Step 3Once smooth, add the chocolate, stirring gently until it's melted and smooth. Scrape the mixture into a large bowl and set it aside to cool to room temperature. Once it's no longer warm, whisk in the egg yolks.
Step 4In a separate bowl, whip the egg whites until stiff. Fold one-third of the whipped whites into the chocolate mixture, sprinkling in the flaky salt. Fold in the remaining beaten egg whites just until no streaks of white remain.
Step 5Divide the mousse into serving glasses, or transfer it to a decorative serving bowl and chill for at least 8 hours. Although it might be tempting to serve this with whipped cream, I prefer it pure, straight up with just a spoon.