Preheat oven to 350 degrees F. With a long spoon, mix butter into the mashed bananas in a large mixing bowl. Mix in the sugar, egg, and vanilla. Sprinkle the baking soda over the mixture and mix in. Add the flour last, mix. Pour mixture into a buttered 4x8 inch loaf pan. Cook for 1 hour.
Let cool on rack. Remove from pan and slice to serve.
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