Preheat the oven to 400°F. Grease a standard 12-cup muffin pan or line it with paper liners.

Place the Nutella in a microwave-safe bowl. Microwave for 30–45 seconds, or until softened and easy to stir. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and brown sugar for 2–3 minutes, or until light and creamy.

Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract.

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

With the mixer running on low speed, add one-third of the flour mixture to the butter mixture, followed by half of the milk. Continue alternating the flour mixture and milk, ending with the remaining flour. Mix only until combined.

Gently fold the chopped bananas into the batter.

Divide the batter evenly among the prepared muffin cups.

Top each muffin with about 2 teaspoons of softened Nutella. Use a toothpick or the tip of a knife to gently swirl the Nutella into the top of the batter.

Bake for 20–25 minutes, or until the muffins are golden and a toothpick inserted into the banana portion of the center comes out clean.

Allow the muffins to cool in the pan for 5 minutes. Transfer them to a wire rack to finish cooling before serving.