Assemble your ingredients. Preheat oven to 350ᵒ F.
In large mixing bowl, sift together flours, sugar, baking powder, baking soda, and salt. Add optional ingredients if desired.
In a separate bowl, combine melted vegan butter, mashed banana, 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 50 minutes 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 45 minutes.