Preheat oven to 375 degrees F. Line the frozen tart shells on a large baking sheet while you prepare the pumpkin pie filling.
Place the pumpkin puree, heavy cream, evaporated milk, brown sugar, salt and pumpkin pie spice in the bowl of your stand mixer and beat with the whisk attachment (or use a large bowl and a hand mixer), until smooth. Add in the eggs one at a time, beating in between. Beat for an additional 3 to 5 minutes or until light and fluffy.
Spoon enough pumpkin pie mixture into each tart shell to fill (approximately 2 to 3 tbsp each).
Place the Nutella into a zip close bag and microwave for 20 seconds to warm it up. Snip off the corner of the bag and swirl a little bit of Nutella right on top of each tart. Bake for 20 to 25 minutes. The pumpkin pie will rise up high in the oven and when cooled will settle back down. Let the pumpkin tarts cool before topping with cool whip or whip cream. Keep stored in the fridge until ready to serve.