Grease the pan. For easy removal of the bars, place parchment paper on the bottom and 2 sides of the pan. Grease again and then set aside. Preheat oven to 350°F. Mash the bananas and set aside as you prep the rest of your ingredients.
In a large bowl, add oil, vanilla, mashed bananas, brown sugar, and eggs.
1 cup mashed bananas, 1/2 cup oil, 1 tbsp vanilla, 2 eggs, 3/4 cups dark brown sugar
Add in the flour, baking powder, baking soda, cinnamon, and salt. Using a rubber spatula, gently fold it in until a soft batter forms. It will be runny.
2 cups flour, 1 1/2 tsps baking powder, 1/2 tsp baking soda, 1/2 tsp cinnamon, 1/2 tsp salt
Transfer into the prepared pan and spread evenly.
Take the Nutella and drop it by the teaspoon across the entire pan. Use about 1/2 cup of Nutella total. Then, take a butterknife and run it across the pan gently, creating marble swirls all around.
1/2 cup Nutella
Bake for 22-25 minutes or until the bars come out clean with a toothpick inserted in the middle.
Allow to cool, slice, and enjoy!