Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease square springform pan (24 x 24 cm; alternatively round, 26 cm Ø). Break chocolate into pieces for the dough.
Melt with 250 g butter in pieces in a saucepan at low heat.
Battez les œufs, le sucre et une pincée de sel avec les fouets du batteur pendant 3-4 minutes jusqu'à obtenir une crème. Retirer le beurre au chocolat du feu et l'incorporer immédiatement au mélange d'œufs.
Mix flour, cocoa and baking powder. Quickly stir into the chocolate-egg mixture. Put the dough into the mould and smooth it down. Bake in a hot oven for about 13-15 minutes.
In the meantime, briefly bring the fruit spread to the boil. Beat the egg white and lemon juice until stiff, adding sugar. Fold in fruit spread in streaks. Pour into a piping bag with a medium perforated spout.
Take the brownie out of the oven. Turn down the oven temperature (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer). Spray egg whites in small tuffs next to each other. Bake the brownie for another 10-12 minutes.
Remove and let it cool down. Remove from the mould and cut into approx. 4 x 4 cm pieces.