In a saucepan, dissolve the caster sugar with 2 tbsp water to make a caramel. When it is ready, add the hazelnuts and mix thoroughly. When the hazelnuts are well coated with caramel, place them on a sheet of baking parchment and leave to dry.
In a saucepan, melt the plain chocolate cut into pieces with the cream and Nutella®. Mix thoroughly until well combined then remove from the heat and leave to cool.
Use a teaspoon to scoop up a nut-sized piece of the chocolate mixture. Add a caramel-coated hazelnut and roll in the palm of your hand to form a small ball. Repeat until all the mixture has been used, then roll the balls in the toasted, chopped hazelnuts. Refrigerate for 30 minutes before serving.