Preheat the oven to 350°F (175°C). Grease a 9x5 loaf pan or line it with parchment paper.

In a large bowl, mash the ripe bananas until smooth.

Mix in the melted coconut oil (or butter), then crack in the egg and add the vanilla extract. Stir to combine.

Sprinkle in the baking soda and salt, then mix well.

Add the sugar and mix until well incorporated. Gradually fold in the flour until it is just blended.

In a separate bowl, combine the brown sugar and cinnamon.

Pour half of the banana batter into the prepared loaf pan, sprinkle half of the cinnamon mixture over it, and layer the remaining batter on top.

Finish by sprinkling the rest of the cinnamon mixture on top and use a knife to swirl the mixture gently through the batter.

Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

Let it cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.