Preparation time: 15 minutes
Baking time: 15-25 minutes
 

1. Melt the chocolate with the water in a bowl set over a pan of hot water (bain marie).
2. Beat the sugar and egg yolks until thick and pale, add the butter, and beat again for a few moments. Carefully incorporate the cooled chocolate. Combine the flour and baking powder and fold it into the mixture. Gently fold in the beaten egg whites.
3. Pour the batter onto a parchment-lined baking sheet and smooth out to a thickness of about 1.5 cm.
4. Bake for 15-25 minutes in the centre of a preheated 160° C (325° F) oven. While still hot, cut into squares.