Position oven rack to the lower-middle position and heat oven to 350 degrees F. Grease bottom and sides of a 9- by 5-inch loaf pan; dust with flour, tapping out excess flour. Spread walnuts evenly on a baking sheet and toast until fragrant, about 5-10 minutes. Set aside to cool. In a large bowl, whisk together the flour, sugar, baking soda, salt, and walnuts. Set aside until needed. In a medium bowl, using a wooden spoon, mix together the mashed bananas, yogurt, eggs, butter, and vanilla. Lightly fold the banana mixture into the dry ingredients with a spatula until just combined and the batter looks thick and chunky. Scrape the batter into the prepared loaf pan. Bake at 350 degrees for 55-60 minutes or until loaf is golden brown and a toothpick inserted in the center comes out clean. Let cool in the pan for 5 minutes, then transfer to a wire rack. Serve warm or at room temperature. This recipe makes one 9-inch loaf.