Put the egg whites with a pinch of salt in a deep container and beat them to snow.
Melt the chocolate in a water bath.
Beat the sugar and egg yolks, then add the cocoa.
Pour the melted chocolate in a thin trickle to the cocoa mixture and beat well.
Add the egg whites in several lots, while stirring well with a spatula. Pour the mixture into a 8 3/4″ (22 cm) x 6″ (15 cm) baking form covered with baking paper.
Bake in a preheated 293°F (145 °C) oven with the fan on for 45 min. or until the cake begins to separate from the walls of the form and hardens in the middle.