Preheat oven to 350°F. Line a muffin pan with 12 paper liners.

In a large bowl, combine the mashed bananas and melted butter, stirring until well combined. Whisk in the brown sugar, granulated sugar, eggs, vanilla extract, and Greek yogurt until smooth.

In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.

Gently fold the dry ingredients into the wet ingredients until just combined, leaving no white streaks. Do not overmix.

Use an ice cream scoop or spoon to fill each muffin liner about ⅔ to ¾ full.

Warm the Nutella in the microwave for 15-20 seconds until it's pourable but not hot. Spoon 1-2 teaspoons of Nutella to the top of each muffin.

Use a toothpick or knife to gently swirl the Nutella into the batter, making circular motions to create swirls.

Bake for 19-22 minutes, or until a toothpick inserted in the center comes out clean and the tops are golden brown.

Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack. Serve warm or at room temperature.