rounded 1/4 teaspoon fleur de sel or flaky sea salt
6oz.(170 g) semisweet chocolate, finely chopped
Mise en place: put a mesh strainer over a large bowl and place the fleur de sel and finely chopped chocolate nearby. Measure out the butter and cream and place them near the stove. Have a silicone spatula, whisk and cooking spoon ready.
Spread the sugar over the bottom of a wide, heavy-bottomed saucepan. Heat the sugar over medium heat. As it liquifies, stir very gently with a silicone spatula to ensure it caramelizes evenly. Watch for it to start to smoke and when tiny black bits start to form, remove from the heat and quickly whisk in the butter until melted. Gradually whisk in the cream and stir until the caramel has dissolved completely. Pour the mixture through the strainer into the large bowl. Remove the strainer and stir in the fleur de sel. Add the chocolate and stir the mixture until the chocolate is completely melted and smooth. Let cool to room temperature.
When the chocolate mixture is no longer warm, whisk in the egg yolks. In a separate bowl, beat the egg whites until stiff. Fold 1/3 of the beaten egg whites into the chocolate mixture then fold in the remaining egg whites until no white streaks remain.
Transfer mousse to ramekins, serving glasses or a serving bowl and chill for at least 8 hours.
Slightly adapted from David Lebovitz's recipe for Salted Butter Caramel-Chocolate Mousse found on epicurious.