Put the condensed milk in a pot and bring it to a boil, stirring constantly to prevent burning. Add the crushed chocolate and stir until homogeneous. Add the butter and chopped walnuts and stir until slightly thickened.
Optionally, you can add dried fruits of your choice. Put the mixture into a small rectangular form and smooth it out. Cover the top with foil and put the form in the refrigerator for several hours. Once the chocolate magic solidifies, cut it into squares and serve it.