Set a rack in the center of the oven and preheat to 350 F.
Prepare a muffin tin by brushing with melted butter or line with muffin cups.
Stir together the rice flour, sugar, corn starch, cinnamon, ginger, baking powder, baking soda and salt. In a separate bowl, whisk together the oil, eggs, banana, vanilla and rum, if using.
Make a well in the center of the dry mixture, add the banana mixture and stir together. Stir in 2/3 of the walnuts.
Divide the batter among the prepared muffin tin and sprinkle the remaining walnuts over the top. Bake for about 20 - 22 minutes, or until the muffin springs back when pressed gently with your finger.
Makes 12 muffins. Recipe by Mani Niall.