No need for a mixer for this recipe. Preheat the oven to 350°F (175°C). 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 last, mix. Pour mixture into a buttered 4x8 inch loaf pan. Bake for 1 hour. Cool on a rack. Remove from pan and slice to serve.