Preheat the oven to 350 degrees F and grease a large ramekin with butter. Mine is 10 ounces to the rim and ~4.25 inches wide. You can also add a circle of parchment paper to bottom for even easier removal.
Mash the banana with a fork in a medium bowl. Mix in the sugar with the fork until well combined.
Add the melted butter, egg yolk, and vanilla - mix well.
Stir in the flour, cocoa powder, baking soda, and salt until just combined. Fold in the chocolate chips.
Pour batter into the greased ramekin. I like to add a few extra chocolate chips on top.
Bake for ~27-32 minutes until set 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.
