Step 1 — Prepare the Oven and Pan
Preheat oven to 350°F (175°C). Grease or line a 9x5-inch loaf pan with parchment paper.
Step 2 — Make the Batter
In a large bowl, whisk mashed bananas, melted butter, brown sugar, eggs, and vanilla until smooth.
In a separate bowl, combine flour, baking soda, and salt. Gently fold dry ingredients into wet until just combined. Do not overmix.
Step 3 — Create the Cinnamon Swirl
Mix brown sugar, cinnamon, and melted butter in a small bowl.
Step 4 — Assemble the Bread
Pour half the batter into the pan, sprinkle half the cinnamon mixture, then add the remaining batter. Drizzle the rest of the cinnamon mixture on top.
Use a knife to gently swirl in a figure-eight motion to create marbling.
Step 5 — Bake
Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean.
Cool for 15–20 minutes before slicing to help set the swirl and keep the texture moist.