Preheat the oven to 350°F (175°C). Melt butter in microwave in a large glass bowl. Add the bananas, mash with back of spoon, and mix to combine. Mix in the sugar, egg, vanilla and cinnamon. Sprinkle the baking soda and salt over the mixture and mix in. Add in the flour.Pour mixture into a greased/sprayed/nonstick 4x8 inch loaf pan.