Finely chop the dark chocolate.
Pour the heavy cream into a saucepan and heat until the edges just start to simmer.
Turn off the heat and add the chopped chocolate, stirring until completely melted.
Add unsalted butter and mix until smooth.
Pour the mixture into a square container, level it, and refrigerate for at least 2 hours until set.
Remove the firm chocolate and cut it into bite-sized pieces.
Dust with cocoa powder.