Preheat your oven to 350°F (175°C). Generously grease a 9x5 inch loaf pan with butter or non-stick spray.
In a large mixing bowl, use a fork to mash the ripe bananas until they are smooth and creamy.
Pour the melted butter into the mashed bananas, stirring until completely blended together.
Add in the baking soda and a pinch of salt; mix well to incorporate.
Stir in the granulated sugar, beaten egg, and pure vanilla extract, mixing until all ingredients are evenly combined.
Gradually add the all-purpose flour to the mixture, folding it in gently with a spatula to avoid overmixing; the batter should remain slightly lumpy.
Pour half of the banana batter into the prepared loaf pan, spreading it evenly with a spatula.
Spoon dollops of Nutella over the banana batter, then use a knife to gently swirl it throughout for a marbled effect.
Carefully pour the remaining banana batter on top, followed by additional spoonfuls of Nutella. Use the knife again to create a gentle swirl on the surface.
Place the loaf pan in the center of the preheated oven and bake for 50-60 minutes. The bread is done when a toothpick inserted into the center comes out clean or with a few moist crumbs attached.
Once baked, allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
Notes
Serve warm or at room temperature. Dust with powdered sugar or drizzle additional Nutella for an extra special touch.