Preheat oven to 350 degrees F (175 degrees C). Grease 18 muffin cups or line with paper baking cups. Stir together the sugar, flour, ground hazelnuts, cocoa powder, baking powder, baking soda, and salt; set aside.
Beat together the eggs, vegetable oil, milk, and vanilla extract 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 center comes out clean, about 15 minutes. Cool in the pan for 10 minutes, then remove from the pan, and allow to cool completely on a wire rack. Once cool, frost with the chocolate-hazelnut spread, and sprinkle with chopped hazelnuts to garnish.
