Mix a small portion of the cold water with the cornstarch and set it aside.
Heat the rest of the water to a simmering boil.
Slowly pour the water and starch mixture into the hot water and bring it to a boil.
Use immediately.
Beat the egg whites with all the sugar.
Weigh out the amount of hot liquid starch required for the recipe.
Whisk it into the unmelted chocolate to make a ganache.
Add the egg yolks and mix.
Add the larger portion of cornstarch and mix again.
When the ganache is between 95°F and 105°F (35/40°C), gently fold in the beaten egg whites using a spatula.
Use immediately.
Baking
16 to 20 minutes at 340°F (170°C) for 800 to 1,000g of sponge in a 40 × 60cm sheet