This recipe is a creation of Heston Blumenthal, famous English chef as well as tv celebrity and author of many cooking books.
Finely mince 180 grams of dark chocolate (70%) and put it in a bowl.
Boil 210 ml of water and pour it gradually on the minced chocolate, while blending with a silicone spatula until obtaining a uniform and creamy mixture. Now put the bowl in another container with ice cubes and beat the mousse with an electrical whisk until the chocolate thickens (don't overdo it otherwise you will ruin the mousse)
Remove the bowl from the ice cubes and keep beating the mousse; it should become lighter and foamy. At this point, divide your chocolate mousse in individual glasses and serve immediately.