Melt half the chocolate into the microwave.
In a bowl beat butter and icing sugar.
Add to the butter mixture eggs, one at time, then melted chocolate ( it mustn’t be hot).
Add salt and sifted flour and cocoa, mix avoinding lumps.
Add, at last, chopped chocolate.
Line a baking tray with baking paper, pour the mixture, spread it evenly and cover with tin foil.
Bake at 356°F for about 20 minutes removing tin foil the mast 5 minutes.
Wait until cold to remove from tray and to cut.
Serve in squares.