Put the chocolate and butter in a bowl and dissolve together, either in the microwave or in a water bath (boil water in a pot and place the bowl of chocolate and butter in it until they dissolve). Mix thoroughly and allow to cool with occasional stirring.
Take 5 tablespoons of powdered sugar and set aside.
Whip the rest of the sugar together with the yolks.
Mix the flour, baking powder, soda, and salt.
Mix the egg whites and the rest of the sugar, and whip them until soft peaks form.
Pour chilled chocolate into the yolk mousse and mix well. Then add ground nuts and flour. Finally, mix bit by bit of the egg whites into the mixture.
Pour the dough into the greased baking mold (butter it and sprinkle with flour). Bake in a preheated oven at 350F for about 35-40 minutes.
Place the chocolate in a bowl and pour hot heavy cream in. Mix thoroughly with a fork.
With occasional stirring, let the icing cool down.
Pour the icing over the chocolate bread and garnish with chopped nuts.