Preheat the oven to 350°F. Butter a 9" x 5" x 3" loaf pan. Combine the bananas, yogurt, butter, eggs, and vanilla in a large mixing bowl, stirring to blend. In a separate bowl, mix together the flour, sugar, walnuts, baking soda, baking powder, cinnamon, and salt. Gently fold the dry ingredients into the wet banana mixture, and stir until fully incorporated. Scrape the batter into the prepared pan. Bake on a low oven rack for about 50 minutes, 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.