Preheat oven to 350°F. Line a small brownie pan or baking dish with a small amount of oil.
Place melted butter, pumpkin, and both sugars in the bowl of an electric mixer fitted with the paddle attachment. You can also use a hand mixer. Mix all together until smooth, about 30 seconds. Mix in egg yolk, baking soda, salt, spices, and vanilla. Mix in flour until batter is smooth.
Press half the batter mixture in the bottom of the pan. Just enough so it makes a thin layer that covers the pan. Bake for 8 minutes.
Place Nutella in a microwave safe bowl. Heat for just a few seconds until pourable. Pour evenly over the top of the partially baked crust. Spread it carefully so it doesn't rip the cookie bars. Scoop tablespoons of batter over the top and carefully spread with your fingers. It won’t be a smooth layer but will mostly cover.
Bake an additional 17-20 minutes until the top is no longer glossy and the edges are golden brown. Cool completely before slicing.