Preheat your oven to 400°F (200°C). Line a standard 12-cup muffin tin with paper liners. This recipe uses the initial high heat method for a nice muffin dome.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, light brown sugar, baking soda, salt, and ground cinnamon (if using) until all dry ingredients are well combined and there are no lumps.

In a separate medium bowl, mash the very ripe bananas using a fork or potato masher until mostly smooth with a few small lumps remaining. Stir in the lightly beaten egg, whole milk, vegetable oil, and vanilla extract until just combined. Do not overmix this wet mixture.

Pour the wet ingredients into the dry ingredients. Using a rubber spatula, gently fold the ingredients together until just combined. Be careful not to overmix; a few streaks of flour are acceptable. Overmixing can lead to tough muffins.

Divide about two-thirds of the muffin batter evenly among the 12 prepared muffin cups.

Spoon approximately 1 teaspoon of room temperature Nutella into the center of the batter in each muffin cup.

Divide the remaining muffin batter evenly over the Nutella in each cup, covering it slightly.

Spoon another 1/2 teaspoon of Nutella on top of the batter in each cup. Using a small spoon or the tip of a butter knife, gently swirl the Nutella into the top layer of batter a few times, creating a marbled effect.

Place the muffin tin into the preheated 400°F (200°C) oven and bake for 5 minutes. After 5 minutes, reduce the oven temperature to 375°F (190°C) without opening the oven door, and continue baking for another 13-17 minutes, or until a toothpick inserted into the center of a muffin comes out with moist crumbs, not wet batter.

Allow the muffins to cool in the muffin tin for 5 minutes before carefully transferring them to a wire rack to cool completely. This helps prevent them from becoming soggy.