Preheat oven to 350°F (175°C) and grease a 9x5 inch loaf pan with butter or non-stick spray.
In a large mixing bowl, combine mashed bananas with the melted butter.
Stir in the brown sugar, beaten egg, and vanilla extract until well combined.
Whisk together the flour, baking soda, and fine sea salt in a separate bowl, then fold into the wet ingredients until just combined.
Pour half of the batter into the prepared loaf pan.
Drizzle 1/4 cup of salted caramel sauce over the batter and swirl gently with a knife.
Top with the remaining batter.
Drizzle the remaining 1/4 cup of salted caramel sauce over the top and swirl again.
Bake for 60 to 65 minutes, or until a toothpick inserted into the center comes out clean.
Remove from oven and immediately sprinkle flaky sea salt over the top.
Allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.