285 grams bittersweet chocolate (about 10 ounces), roughly chopped, more as needed (see note)
Fleur de Sel (or sea salt), to taste
1. Create an ice bath in a large bowl using ice and a little cold water. Nestle a smaller bowl in ice bath.
2. Place chocolate and 1 cup water in a small pot and heat over medium. Whisk until mixture is melted and smooth, about 3 to 5 minutes.
3. Immediately pour melted chocolate into the bowl in the ice bath. Vigorously whisk chocolate mixture by hand until thick, 3 to 5 minutes. The chocolate should be fluffy and form a mound when dolloped with the whisk (it should generally have the texture and appearance of mousse). If the mixture does not thicken, add a bit more chopped chocolate and remelt over the heat. Spoon into serving bowls and garnish with fleur de sel.
Note: For a lighter mousse, use 265 grams of bittersweet chocolate or just add more water.