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 4×8 inch loaf pan. Bake for 1 hour.
Cool on a rack. Remove from pan and slice to serve.