Preheat oven to 350 degrees. With a wooden spoon, mix butter into the mashed bananas in a large mixing bowl. Mix in the sugar, egg, and vanilla. Sprinkle the baking soda and salt over the mixture and mix in. Add the flour and mix. Pour mixture into a butter 4 x 8 inch loaf pan. Bake for 1 hour. Cool on a rack. Remove from pan and slice.