Preheat the oven to 350°F/180°C. Line a square 8×8 baking pan with parchment paper and set it aside.
In a large bowl, add the melted butter, pumpkin, dark brown sugar, and white sugar. Whisk until well combined.
1/2 cup butter, 1/3 cup pumpkin puree, 3/4 cups dark brown sugar, 1/3 cup white sugar
Add in egg yolk and vanilla and continue whisking.
1 egg yolk, 1 tbsp vanilla
Add in the flour, baking soda, cinnamon, salt, and allspice. Using a spatula, gently fold the ingredients in. Don't over-mix!
2 cups all-purpose flour, 1/2 tsp baking soda, 1 tsp cinnamon, 1/2 tsp salt, 1/2 tsp allspice
Place 1/2 of the dough into the prepared pan. Make sure it's covering the entire bottom of it.
Take spoonfuls of Nutella and drop it on top. It won’t be a smooth layer but should cover most of the dough. Take the remaining cookie dough and press it on top. It's also okay if there is Nutella peeping through! See photos above for reference.
2/3 cups Nutella
Bake an additional 20-25 minutes. The edges will be a golden brown. Cool completely before slicing. Enjoy!