1. Preheat oven to 350 degrees. Spray a 9-by-13-inch baking pan with cooking spray. Set aside.
2. In a medium bowl, whisk together flour, cinnamon, nutmeg, baking soda, and salt. Set aside.
3. In the bowl of a stand mixer, cream butter and sugars until smooth. Beat in the egg and vanilla until well blended. Add the pumpkin puree. Reduce speed to low, and mix in the flour mixture until just combined.
4. Spread batter evenly into the prepared pan. Drop tablespoons of Nutella all over the pumpkin cake batter. Swirl in the Nutella with a knife. I wanted the Nutella to stay towards the top, to act as the frosting, so I swirled gently.
5. Bake for about 35-40 minutes or until the sides pull away from the pan and a toothpick comes out clean. Place cake on a wire rack and cool completely. Cut into squares and serve.