Preheat oven to 425F and line muffin tin with muffin liners or parchement paper
In a large bowl mix together the flour, baking powder, baking soda, cinnamon and salt.
In a medium bowl whisk together the oil, white and brown sugars, eggs, milk, vanilla and mashed banana.
Add the wet ingredients to the dry ingredients and mix together gently with a wooden spoon or spatula just until all the ingredients are combined.
Add 2/3 cup of the chopped walnuts if using and mix until just combined.
Pour the batter into the 12 paper liners and sprinkle the remaining walnuts overtop if using.
Bake for 5 minutes at 425F then reduce the temperature to 375F and bake for another 15 minutes or until a toothpick inserted into the center comes out clean.