Preheat the oven to 350°F with a rack in the center. Lightly grease an 8" square pan, then line it with a parchment sling.

In a large saucepan, mash the banana with the sugars until well combined. Bring the mixture to a boil over medium heat, then cook, stirring frequently, until shiny, syrupy, and caramelized, about 4 to 8 minutes.

Remove the pot from the heat, let cool for about 5 minutes, then add the butter, yolk, and vanilla. Stir until creamy and emulsified.

Whisk in the salt and baking powder, then stir in the flour until only a few dry streaks remain. Let the batter cool for about 5 minutes, then stir in the nuts and chocolate.

Transfer the thick, soft batter to the prepared pan and spread into an even layer. Sprinkle with flaky salt if desired.

Bake for 20 to 22 minutes, until the bars are set in the center and a toothpick comes out with no raw batter.

Let cool completely in the pan (at least 2 hours) before transferring to a cutting board to slice and serve.

Store the banana bread bars in an airtight container at room temperature for up to 4 days or freeze for longer storage.