Preheat oven to 400 degrees F. Grease a 6 portion muffin tin or line with muffin papers.
In a bowl, combine the flour, baking powder, baking soda, salt and both types sugar. Stir well.
In a second bowl, stir or whisk together the egg and yogurt, then stir in mashed banana and vanilla.
Combine flour mixture and egg mixture and stir until evenly blended, then add oil (or butter oil mixture) and stir until blended without overbeating. Batter should be thick. Stir in walnuts if using
Divide batter among the muffin cups, piling toward the center.
Put the muffins in the oven and set timer for 5 minutes. Bake at 400 for 5 minutes.
Reduce oven heat to 375 degrees F and continue baking for 13 minutes for a total bake time of about 18 minutes.