With oven rack adjusted to the middle position, preheat oven to 350℉. Spray loaf pan with nonstick cooking spray and line with parchment paper. In a medium sized bowl, whisk together flour, baking powder, salt, and baking soda.
In a large mixing bowl, use a hand mixer set to medium speed. Beat together butter and sugar until light and fluffy. Add eggs one at a time until incorporated.
In another medium sized bowl, stir together mashed bananas, milk, and vanilla extract.
With mixer turned to low, alternate mixing in flour and banana mixture. Starting and ending with flour mixture. Mix just until combined scraping sides of bowl as needed.
Transfer half of banana bread batter into loaf pan. Swirl 1/3 cup of Nutella into batter. Top with remaining batter and finish by swirling remaining 1/3 cup of Nutella on top. Bake for about 1 hour and 10 minutes. Let loaf cool in pan for about 15 minutes, then use parchment paper to remove banana bread from pan and let cool on wire rack.