Grease a 9x5 loaf pan with oil or butter. Set aside.
Preheat the oven to 350 degrees F (177 degrees C).
Brown your butter. Heat butter over medium heat until it fully melts. Lower to low heat and cook until the butter turns a golden yellow color. Remove from the heat and set aside. The butter will continue to brown as it sits.
Add all purpose flour, baking soda, and cinnamon to a bowl. Mix until well combined. Set aside.
Add the brown sugar, eggs, and browned butter to a separate bowl or the bowl of a stand mixer. Whisk until you get a light, fluffy mixture (1-2 minutes).
Add the smashed bananas, vanilla extract, and salt. Mix until combined.
Lastly, add your dry ingredients from step 4 to the wet ingredients. Mix until just combined. Be careful not to over mix the batter.
Spoon half the batter into the prepared loaf pan. Drop spoonfuls of the nutella on top of the batter. You will want to use about half the nutella. Use a knife to swirl the nutella into the batter. Repeat this process for the last half of the batter and nutella.
Bake for 55-60 minutes until a toothpick comes out clean.
Let it rest for 15-20 minutes before cutting into it.