Heat the heavy cream on the stovetop or microwave until simmering.
Pour the heavy cream over the chopped dark chocolate.
Wait a few minutes for the chocolate to melt, then stir to mix.
Pour the chocolate into a shallow pan and press cling film directly on top of the surface. Put in the fridge to set, at least one hour.
When the chocolate is firm, but soft, divide into 8 spoonfuls. Place the spoonfuls of chocolate onto a nonstick surface like cling film or a silicone mat and put back in the fridge to harden a little more, 20-30 more minutes.
Roll the spoonfuls into balls with your hands. Roll the chocolate balls in cocoa powder or sprinkles to coat. If time allows, put the truffles in the freezer before making the daifuku.