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, place parchment paper on a cutting board. (You may want to use aluminum foil instead.)
With a big spoon or ladle, transfer the chocolate paste to the parchment paper.
Holding both sides of the parchment paper, shape the paste into a chocolate “sausage” and wrap it up.
Put it in the freezer for 2 hours.
Once the chocolate base becomes firm, remove it from the freezer. Put it on the cutting board and slice it.
Serve cold. Put leftovers into the freezer.