Preheat the oven to 350 degrees F / 175 degrees C. Generously grease two 8 by 4-inch / 20 by 10cm loaf pans, then line them with greased parchment paper.

In a large bowl, combine the bananas, brown sugar, butter, milk, eggs, and vanilla and mix well with a wooden spoon.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry ingredients to the banana mixture and stir just until smooth.

Heat the Nutella in the microwave for about 20 seconds to soften.

Divide half of the banana batter between the prepared pans. Drizzle half of the Nutella over the two pans and run a knife through the batter in a zigzag pattern to distribute.

Add the remaining batter to the pans, top with the remaining Nutella, and swirl into the batter with the knife again.

Bake until a tester comes out clean, about 50 minutes. Let the cakes cool in the pans for about 10 minutes, then remove from the pans and set on a wire rack to cool completely before serving.