Melt together dark chocolate and the butter. Set aside to cool. Beat fluffy cream eggs and sugar. Add vanilla and a pinch of salt. Sift together the flour, cocoa, baking powder and baking soda. With stirring gently add the flour and cocoa mixture to the egg, and then the milk. Finally put the butter and chocolate. Pour the dough into a greased and sprinkled with flour cakes rectangular shape. Break the white chocolate into pieces and Pierce the dough on a surface so that its edges are visible. If you drop a very deep baking will fall to the bottom. Bake the cake in a preheated 170 C oven.