Preheat oven to 350. Line a 12-count standard muffin pan with baking cups.
In a mixing bowl, whisk together the pumpkin, sugar, eggs, oil, water, and vanilla until well-combined. In another bowl, mix together the flour, baking soda, salt, and pumpkin pie spice. Incorporate into the wet ingredients, mixing until smooth. Pour batter into the baking cups, filling about 2/3-3/4 full.
Put the Nutella into a microwave-safe bowl and microwave for 20-30 seconds or until it thins out just enough to swirl. Using a teaspoon, drop a spoonful or two of Nutella onto the top of each muffin and swirl with a knife.
Bake for 25-30 minutes or until set. Best served warm.
Notes

If you would like Nutella in the middle of the muffin also, you can fill the cups half way, add a spoon of Nutella, top with more batter, then swirl the top with more Nutella as directed (this method will increase the Nutella measurement required for the recipe).