Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper, leaving an overhang on the longer sides for easy removal.

In a large mixing bowl, mash the ripe bananas thoroughly with a fork or potato masher. Add the melted butter, egg, milk, and vanilla extract. Whisk until well combined.

In a separate medium bowl, whisk together the flour, granulated sugar, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing with a rubber spatula until just combined. Be careful not to overmix; a few lumps are fine. Overmixing can lead to tough banana bread.

Pour about half of the banana bread batter into the prepared loaf pan. Dollop about half of the warmed Nutella over the batter. Using a knife or a skewer, gently swirl the Nutella into the batter. Do not overmix, just create a nice swirl pattern.

Pour the remaining banana bread batter over the Nutella layer. Dollop the remaining Nutella on top and repeat the swirling process.

Bake for 50-65 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf (avoiding large Nutella pockets) comes out clean or with moist crumbs, but no wet batter.

Once baked, remove the loaf pan from the oven and let it cool in the pan for 10-15 minutes. Then, carefully remove the banana bread from the pan and transfer it to a wire rack to cool completely before slicing and serving. This helps prevent it from becoming gummy.