Preheat the oven to 350 degrees. Butter a 9" x 5" x 3" loaf pan.
Combine the bananas, yogurt, butter, eggs, and vanilla in a large mixing bowl. Stir to blend.
In a separate bowl, mix together the flour, sugar, walnuts (if using), baking soda, baking powder, cinnamon, and salt. Gently fold the dry ingredients into the wet banana mixture and stir until fully blended.
Scrape the batter into the prepared pan.
Bake on a low oven rack for about 50 minutes or until a toothpick inserted into the center of the bread comes out clean. Let cool for 5 minutes in the pan. Eat warm or at room temperature.