Add the muffin liners to your baking tin, or spray with nonstick baking spray.
Make the batter
In a medium size bowl, add the flour, baking soda, salt and cinnamon, and whisk until combined. Set aside.
Peel your bananas and put them into a medium bowl. With a large fork, mash them up and set aside.
Add your butter and brown sugar to a large bowl or the bowl of a stand mixer. Beat on medium-high speed for 3-5 minutes, until it is light and fluffy.
Add the egg and beat until fully incorporated.
Add the mashed bananas, sour cream and vanilla to the butter mixture and beat until fully combined.
Add the flour mixture in three sections, mixing just a little bit in between. On low speed, mix until just combined. Do not overmix.
Bake
Scoop the batter into the prepared tin. I use an ice cream scoop to get equal sized muffins.
With a tablespoon or similar sized spoon, scoop a dollop of Nutella on top of each unbaked muffin. Swirl the Nutella around using a toothpick or skewer.
Bake for 20-25 minutes, until the tops are set or the internal temperature reads 205°F (96°C).
Let the muffins cool in the pan for about 10 minutes, and then move them to a cooling rack to cool the rest of the way.
Store in an airtight container for up to 2 days, or in the fridge for a week. These also freeze very well in a zip top bag.