8 ounces dark (bittersweet) chocolate, with a minimum 55 percent cocoa, and preferably between 60 and 65 percent, finely chopped (may use one 8.8-ounce bar, such as Valrhona 62%)
1/3 to 1 cup water (may substitute other liquid, such as fruit juice)
2/3 cup heavy cream, whipped with 2 tablespoons sugar or to taste (optional)
Combine the chocolate and water in a medium stainless-steel mixing bowl.
Fill a saucepan (just large enough to cradle the bottom of the mixing bowl) with ice cubes and water. Set aside.
Heat some water in a separate saucepan (just large enough to cradle the bottom of the mixing bowl) over medium heat. Remove it from the heat and place the bowl over the saucepan; stir just until the chocolate has melted and the mixture is shiny and smooth, with the water fully integrated.
Seat the bowl atop the ice water-bath saucepan; use a large whisk and start whisking vigorously. For the first couple of minutes not much will happen, and there will be a viscous, runny mixture. Then, often quite quickly, the chocolate will start to firm up to whipped-cream consistency (within 4 to 5 minutes); it may be best to remove it from the cold bath while the mixture is still a little loose and continue whisking until thick and creamy. If it will not firm up, return it to the ice-water bath.
If the mixture sets and becomes grainy (closer to a ganache), re-position the bowl atop the heated water for a few seconds to warm the mixture, then whisk to the desired consistency.
To serve, divide among individual dishes and top with a dollop of whipped cream and passion fruit seeds, if desired.
The “chantilly” (not made with whipped cream, as traditional chantillys are) can be enhanced with the addition of a flavored liquid, such as orange juice or orange liqueur, lapsang souchong tea or malt whisky. But the amount of water must be reduced accordingly. It is also nice to flavor the chocolate with spices such as vanilla, anise, chili powder or fennel pollen.
The precise ratio of water to chocolate depends on what kind of chocolate you use, the temperature of the ice water bath and, to a lesser degree, the temperature in the room. As always, when you are trying out a new recipe and a new technique, something may go wrong. But there are an unlimited number of chances to perfect this recipe; add more water or chocolate to adjust the viscosity. For a thick consistency, use the minimum amount of water called for. But be aware that the time between a lovely, whipped-cream texture and a rather grainy one is short. If you want to experiment, use the maximum amount of water, but be prepared to add more chocolate if the chantilly refuses to firm up.
Use a chocolate that is between 60 and 70 percent cocoa. Because the chocolate flavor is intense, a bit of whipped cream and a few passion fruit seeds provide perfect complements.
The chantilly can be made a few hours in advance and will firm up slightly, whether refrigerated or at room temperature. For best results, reheat in the double-boiler/bowl method described below and whisk to the desired consistency just before serving.