Preheat your oven to 350°F (175°C).
Grease your loaf pan with butter or cooking spray. Optionally, line it with parchment paper.
In a large mixing bowl, combine the softened butter and canola oil. Add the granulated sugar and whisk until light and fluffy. Stir in the mashed bananas, eggs, and vanilla extract until well incorporated.
In another bowl, whisk together the all-purpose flour, baking powder, and salt.
Gradually add the dry mixture to the wet ingredients, alternating with the buttermilk. Start and end with the dry ingredients and mix until just combined.
Gently fold in the chopped walnuts with a rubber spatula.
Pour the batter into the prepared loaf pan and smooth the top. Bake for 60-70 minutes or until a toothpick comes out clean.
Once baked, let the loaf cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
