In a medium saucepan over medium heat, bring the cream to a boil.
Break the chocolate into a bowl, then pour in the cream in two batches, mixing with a silicone spatula or whisk between each batch. Add the butter in chunks and mix well.
Line a rimmed baking sheet with parchment paper or plastic wrap. Pour the ganache onto it, resulting in a 1/4-inch (5 mm) thick layer. Chill in the refrigerator until firm, about 1 hour.
Spread the Nutella over the top of the ganache, then roll it up as you would a jelly roll, using the parchment to help.
Cut the roll into 1/2-inch (18 mm) lengths. Sprinkle cocoa powder onto your hands and form each tube into a small ball, then dip them in cocoa powder.