Preheat oven to 180 degrees C. Grease 18 muffin cups or line with patty papers. Stir together the sugar, flour, ground hazelnuts, cocoa powder, baking powder, bicarb soda and salt; set aside.
Beat together the eggs, vegetable oil, milk and vanilla essence with an electric mixer until smooth. Stir in the flour mixture until moistened, then stir in the water until batter is smooth. Batter may be a bit thin. Pour into the prepared muffin cups.
Bake in preheated oven until a toothpick inserted into the centre comes out clean, about 15 minutes. Cool in the tin for 10 minutes, then remove from the tin and allow to cool completely on a wire rack. Once cool, ice with the chocolate-hazelnut spread and sprinkle with chopped hazelnuts to garnish.