In a bowl, combine the mousse powder and water until the powder has dissolved. Stir in the dulce de leche and Fleur de Sel. Whip the cream until stiff and gradually fold in, chill for at least 1 hr.
Boil the sugar and water in a wide-bottomed pan without stirring. Reduce the heat and simmer, shaking the pan occasionally until a light brown caramel has formed. Remove the pan from the heat, add the popcorn, mix, divide onto a sheet of baking paper, allow to cool.
Spoon the mousse onto plates. Roughly chop ¼ of the popcorn and scatter over. Serve with the remainder of the popcorn.
To speed things up:
Instead of caramel popcorn, use ready-made chocolate popcorn.