Set aside 175 g of the chocolate and walnuts. Melt the rest of the chocolate with the butter in a double boiler.
Beat sugar, salt and eggs until frothy, then stir in the chocolate butter mixture. Mix the flour with the baking powder and stir into the egg mixture. Then fold in the chocolate and nut mixture.
Grease a baking or baking dish (approx. 30 cm) and distribute the dough evenly in it. Bake at 180 degrees for about 30 minutes.
The cake should be firm on the outside and a bit mushy on the inside. Let cool and then cut into approx. 12 pieces.