Preheat the oven to 350 degrees.
Prepare the Nutella filling by lining a baking sheet with parchment paper and spooning 1-1.5 tablespoons of Nutella on the sheet (to make 14-16 total dollops). Place in freezer for 15-20 minutes to set.
Using a stand mixer fitted with the paddle attachment or a hand mixer, cream the butter and sugar on high for 1-2 minutes. Scrape down the bowl once complete.
Add in eggs, sour cream and pumpkin puree. Mix until combined. Scrape down the bowl once complete.
In separate bowl, whisk dry ingredients together (flour, pumpkin pie spice, baking soda, baking powder, salt).
Add dry ingredients to wet ingredients in 4 portions, mixing on low-medium for 15-30 seconds between each addition.
Thoroughly grease or line muffin tins (recipe usually makes 12-14 muffins)
Fill half of the tin with pumpkin batter using a cookie dough scoop or a spoon.
Remove the Nutella fillings from the freezer and place one frozen dollop in the center of each muffin tin.
Spoon the remaining pumpkin batter over the top and lightly smooth it out. Fill each tin about 3/4 full.
Generously sprinkle the streusel topping over the tops of the muffins.
Bake for 18-20 minutes, or until a toothpick comes out clean (just test the top portion of the muffins with a toothpick, otherwise you will hit the Nutella).
Remove from oven and let cool for 15-20 minutes before removing the muffins from the pan. Store at room temperature.