Mix together first measure of granulated sugar, salt and egg white powder. Pour over liquid white egg and stir till dissolved. Whip mixture to create a creamy meringue mass.
Separately, beat whole egg, egg yolks and sugars (second measure of granulated and inverted) to obtain a foamy mass
Heat the couverture to 50°C/122°F and fold onto the meringue mass. Add butter.
Sieve flour and cocoa powder and mix well into meringue mass. Fold in foamy egg mass.
Pour the sponge mixture into a 1 cm (0.82in) high silicone tray.
Bake at 155C/311F in a convection oven for 20-25 min, or at 170C/338F in a deck oven for 15-20 min.