Cupcakes: Preheat the oven to 200 degrees C. Line two 12 hole bun trays with colourful paper cases.
Sift the cocoa into a large bowl; add 4 tablespoons of boiling water and mix into a thick chocolate syrup, adding more water if you needed. Add the softened butter, sugar, self-raising flour, baking powder and eggs. Mix with electric beaters until smooth and evenly mixed.
Spoon teaspoons of the mixture into the paper cases, filling them 3/4 full. Bake for 10 to 15 minutes or until the sponge mixture has risen and the cupcakes spring back when you touch them. Remove from the oven then cool on a wire rack.
Buttercream Icing: Beat the softened butter in a large bowl then sift in the icing sugar. Mix with an electric beater until smooth. Add the Nutella and cream then mixing again until evenly distributed.
To Assemble: Spoon the buttercream icing into a piping bag with a number 5 star nozzle. Pipe icing evenly on top of each cupcake. Decorate with chocolate sprinkles, chopped hazelnuts or however you like.