Preheat the oven to 425°F. Line a 12-cup muffin pan with muffins liners. Line it with 6 liners and spread it through the 12 holes. This gives the muffins a much bigger rise.
In a medium bowl, sift the flour. Add the baking soda, salt, and cinnamon. Set aside. Heat the butter in the microwave, let it sit for 10-15 minutes until cooled to room temperature.
2 cups All-purpose flour, 1 teaspoon Baking soda, ½ teaspoon Salt, 1 ½ teaspoon Ground cinnamon, ½ cup Unsalted butter
In a large bowl, mash the ripe bananas with a potato masher or a fork. Mash the bananas well. Add the melted butter, brown sugar, eggs, sour cream, and vanilla. Use a whisk to whisk until smooth and combined.
¾ cup Brown sugar, 2 Large eggs, ¼ cup Sour cream, 3 Ripe bananas, 1 teaspoon Pure vanilla extract
Add the dry ingredients and mix until just combined. Melt the Nutella in a microwave safe bowl in the microwave for 30 seconds until melted.
⅔ cup Nutella
Use a cookie scoop to scoop 1 scoop of batter into the 6 muffins liners. Spoon 1 teaspoon of Nutella on top of the batter. Use a butter knife to lightly swirl it together. Add another scoop of batter on top, the muffin liner will be full. Add another teaspoon of Nutella and swirl it on top.
Bake for 5 minutes at 425°F Turn the oven down to 375°F. Bake for another 10-12 minutes. Bake until a toothpick inserted comes out clean.
Cool the muffins in the hot pan for 10 minutes before transferring to a cooling rack.