Preheat the oven to 350°. Line a 9"x5" bread pan with parchment paper. Set aside.
MAKE THE BANANA BREAD BATTER: Add the all-purpose flour, brown sugar, baking powder, ground cinnamon, and salt to a large mixing bowl. Whisk until well combined. Set aside.
Add the mashed bananas, buttermilk, oil, eggs, and vanilla extract to another large mixing bowl. Whisk until well combined.
Pour the wet ingredients into the bowl with the dry ingredients. Mix with a silicone spatula until just combined. Set aside.
ADD THE CINNAMON SWIRL: To make the cinnamon swirl mixture, add the brown sugar and ground cinnamon to a small mixing bowl. Whisk until well combined. Set aside.
Pour half of the banana bread batter into the prepared baking tin. Sprinkle over the cinnamon swirl mixture, and gently swirl through the batter. Top with the remaining banana bread batter.
BAKE THE BREAD: Bake the banana bread for 45 minutes, or until cooked through. The banana bread is cooked through if it bounces back when lightly touched in the center. If the top of the bread is browning too quickly before the center is done, loosely place foil over the top of the bread until it is done baking. Allow to cool in the tin for 10 minutes before transferring to a cooling rack to cool completely.
ADD THE GLAZE: To make the vanilla glaze, add the powdered sugar, vanilla extract, and 1 tablespoon of boiling water to a small mixing bowl. Whisk until well combined. Drizzle over the cooled banana bread.