Preheat your oven to 350ºF. Grease a 9″x 5″ loaf pan with butter and dust with flour.
In a large bowl, combine melted butter and sugar. Add eggs and vanilla. Whisk to combine.
In a medium bowl, combine flour, baking soda, and salt and stir into the butter mixture until smooth. Fold in sour cream, walnuts, and bananas. Spread evenly into the prepared pan. Bake at 350º for 60 minutes, or until a knife inserted into the center comes out clean.