Assemble your ingredients. Preheat oven to 350ᵒ F.
In large mixing bowl, sift together flours, sugar, baking powder, baking soda, and salt.
In a separate bowl, combine melted butter, mashed banana, buttermilk or soured milk, and vanilla extract. Add to dry ingredients, mixing just until well moistened. Batter should be thick but not dry. Add a little more milk, if needed.
Turn at once into a greased 9 x 5-inch loaf pan. Smooth top (dampening your fingers with water will help). Bake for 1 hour or until a toothpick inserted in the top of the loaf comes out clean.
Let stand in pan 10 minutes, then remove from pan and place on a wire rack. Let cool completely before slicing.
Note : If using a glass loaf pan, reduce the temperature to 325° F to avoid over-browning. Also, start checking it at 50 minutes.