Preheat oven to 350F. Grease a 9x5-inch loaf pan with butter or baking spray. In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the brown sugar and butter together on high speed until light and fluffy, about 3 minutes. Beat in the eggs one at a time, stopping to scrape down the bowl between each. Beat in the vanilla extract. Mix the mashed bananas in and then add the dry ingredients and fold together until just combined. Pour the batter into the prepared loaf pan. Bake for 50 to 60 minutes or until a wooden pick inserted into the center comes out clean. Let cool in the pan for 20 minutes then remove and finish cooling on a wire rack.