Preheat the oven to 350 degrees F and grease a ramekin with butter.
Mash the banana with a fork in a medium bowl. Mix in the sugar and cinnamon with the fork until well combined.
Add the melted butter, egg yolk, and vanilla - mix well.
Stir in the flour, baking soda, and salt until just combined.
Pour into the greased ramekin and bake for 25-30 minutes until golden brown on top and a toothpick comes out with just moist crumbs, no wet batter.
Remove from the oven and cool slightly before eating. The banana bread is easiest to remove from the ramekin when still warm.
