Finely chop the dark chocolate.
Pour heavy cream into a saucepan and heat over medium-low.
Add the chopped chocolate to the warmed cream and melt.
Add butter and mix well.
Line a rectangular container with parchment paper and pour in the chocolate mixture.
Chill in the refrigerator for at least 3 hours until set.
Remove the hardened chocolate, cut it into suitable sizes, and dust with cocoa powder.