Preheat your oven to 350°F (175°C).
In a large mixing bowl, whisk together melted butter and sugar until well combined. Add in eggs, vanilla extract, vegetable oil, and whole milk. Mix until smooth and creamy.
Fold in the mashed bananas until well integrated.
In a separate bowl, combine all-purpose flour, baking soda, baking powder, ground cinnamon, and sea salt. Stir until well mixed.
Gradually add the dry mixture into the wet mixture, folding gently to combine.
Fold in the roughly chopped walnuts.
Divide the batter evenly between prepared loaf pans, smoothing the top with a spatula.
Bake for 60-70 minutes, or until a toothpick inserted comes out clean.
Cool for about 10 minutes, then transfer to a wire rack to cool completely before slicing.