Preheat the oven to 350℉. Line a 9x9 inch pan with aluminum foil and spray with cooking spray. Set aside.
In a large bowl, mix together the butter, pumpkin puree and both sugars until combined. Mix in the egg yolk, baking soda, salt, pumpkin pie spice, cinnamon, vanilla and flour until smooth.
Press half of the batter into the prepared pan and bake in the preheated oven for 8 minutes.
In a microwave safe bowl, microwave the Nutella for 30 seconds. Mix until it is pourable.
Pour the Nutella on the partially baked dough. Blob the remaining dough on top. It doesn't need to cover the Nutella.
Bake for an additional 18 minutes, or until the edges are golden brown. Allow to cool completely in the pan.
Refrigerate before cutting into 16 bars. Store in the fridge for up to 1 week.