Preheat the oven to 425ºF (220ºC). Lightly grease a standard 12 cup muffin tin with 1 teaspoon of butter (or more if needed) then line the tin with muffin cups. Set aside.
Whisk together the flour, baking soda, and salt in a bowl until well combined. Set aside.
Place the butter, brown, and white sugar in another mixing bowl. Use a handheld or stand mixer to beat the butter and sugars together until light and fluffy, about 2 to 3 minutes.
Add the bananas to the bowl of butter and sugar. Use a potato masher or a fork to mash the bananas directly in the bowl.
Add the sour cream and vanilla. Once again, beat the mixture with the handheld or stand mixer until fluffy and well combined.
Dump the flour into the bowl of wet banana mixture. Use a rubber spatula to gently fold everything together until just mixed with a few streaks of flour remaining. The batter will be quite thick.
Use a small cookie scoop or spoon to portion about 1 tablespoon of banana batter into each muffin cup in the prepared tin. Lightly wet a fingertip to gently smooth the top of the batter in the tin.
Place the Nutella in a heatproof measuring cup or bowl. Heat the Nutella in the microwave for 45 to 60 seconds to loosen. Heating the Nutella won’t melt it per se, but will make it smoother and easier to handle.
Use a small spoon to dollop about 1 teaspoon of Nutella in the middle of each muffin cup in the tin.
Divide the remaining banana batter evenly among the 12 muffin cups, and again, gently smooth the top of the batter with a barely wet fingertip.
Drizzle a little remaining Nutella on top of each muffin until all of the Nutella is gone. Use a toothpick to swirl the Nutella on the muffin top if you are feeling a little creative.
Place the muffins in the preheated oven and bake them for 16 to 18 minutes, or until the muffin tops are golden. Keep in mind the Nutella will not melt, but will become smooth and probably look a bit cracked on top of the muffin.
When the muffins are golden, remove the tin from the oven and set it on a wire rack to cool for 10 minutes.
Remove the muffins from the tin and transfer to the wire rack to cool completely.
Once cool, serve immediately, store in an airtight container on the counter for two days, or store them into a reusable large zip top bag in the freezer for up to one month.