Preheat your oven to 350°F (175°C). This ensures your bread will bake evenly.
Grease your loaf pan with a bit of butter or line it with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the large eggs, one at a time, mixing well after each addition.
Stir in the mashed brown bananas, buttermilk, and vanilla extract. Mix until well combined.
In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and baking soda.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake in the preheated oven for 55-65 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, let it cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely before slicing.
