Finely chop the dark chocolate.
Heat the heavy cream until just before boiling.
Add the chopped chocolate to the warmed cream and stir until melted.
Add butter and mix thoroughly until completely melted and combined.
Line a square mold with parchment paper and pour the mixture in, spreading it evenly.
Refrigerate for 3-4 hours until set.
Cut into bite-sized pieces, coat with cocoa powder, and finish.