Prepare muffin tins by lining with paper liners or spraying with a nonstick spray.
In a large mixing bowl, whisk eggs together until foamy.
2 eggs
Whisk in brown sugar, oil, milk and vanilla.
¾ cup brown sugar, ½ cup oil, ¼ cup milk, 1 teaspoon vanilla
Add flour and sprinkle baking powder, baking soda, salt and cinnamon over the top of the flour. Use a silicone spatula or large wooden spoon to fold the ingredients together. Stir just until the flour is mostly mixed in.
2 cups flour, 1 teaspoon baking powder, 1 teaspoon baking soda, ½ teaspoon salt, ½ teaspoon cinnamon
Add mashed bananas and stir.
1 ½ cups mashed bananas
Heat Nutella in the microwave for 10-20 seconds, just until it's runny, but not hot.
⅔ cup chocolate hazelnut spread
Place ¼ cup Nutella in the center of the batter. Use the tip of the spatula to swirl the Nutella into the batter.
Scoop batter into paper-lined muffin tins.
Add about 1 teaspoon of remaining Nutella into the center of each muffin. Use a toothpick to swirl the Nutella into the batter.
Bake in preheated 375°F oven for 18-20 minutes until baked through.
Notes
Use fresh baking powder - baking powder can lose effectiveness; if old, your muffins might not rise properly.
Fully preheat your oven - I use a separate oven thermometer in my oven to make certain my oven has reached the proper temperature. My preheat light often turns off before the oven has reached full temperature so I always give it extra time.
Do not overmix batter - Just stir until everything is mixed in. Over-stirring can cause tough muffins.
Swirl the Nutella - Gently swirl the Nutella into the batter with the tip of a knife or a toothpick. Swirling creates a beautiful pattern.
Use an ice cream scoop - Helps create even-sized muffins. Plus it makes putting the batter in the tins easy peasy!