Preheat the oven to 350°F and generously grease a 9×5-inch loaf pan with butter or baking spray. In a medium bowl, combine the flour, baking soda, salt, and cinnamon, whisking thoroughly to distribute evenly. In a separate large bowl or the bowl of a stand mixer with the paddle attachment, cream the softened butter and brown sugar together on high speed for about 3 minutes until the mixture turns light and fluffy. Incorporate the eggs one at a time, ensuring to scrape down the sides of the bowl after each addition. Add the vanilla, then gently mix in the mashed bananas. Gradually fold in the dry ingredients until just blended, taking care not to overmix. Transfer the batter into the prepared pan, smoothing the top. Bake for 50 to 60 minutes or until a toothpick inserted into the center comes out clean. Allow the bread to rest in the pan for 20 minutes before transferring it to a wire rack to cool completely.