Chop chocolate into smaller pieces and measure. (If you use chocolate chips just measure the amount required for the recipe). Put aside.
Pour condensed milk into a casserole dish with a thick bottom and bring the milk to a boil at medium heat.
Add pieces of chocolate to the condensed milk and melt it, stirring constantly. (Dark chocolate melts at 50 C).
Add butter and nuts to the mixture, reduce the heat, and continue to stir the mixture for 20-25 minutes, until it turns into a sticky paste.
Meanwhile, line a baking dish with parchment paper.
With a big spoon or ladle, transfer the chocolate paste to the parchment paper.
Spread the chocolate paste evenly. It will look like a chocolate cake.
Put it in the freezer for 2 hours.
Once the chocolate base becomes firm, remove it from the freezer.
Transfer the firm chocolate cake to the cutting board and cut it into small squares.
Decorate with fruit, nuts, chocolate chips, etc.
Serve cold. Put leftovers into the freezer.