Preheat your oven to 350°F (175°C).
Grease your loaf pan with butter or line it with parchment paper.
In a mixing bowl, whisk together the all-purpose flour, baking soda, kosher salt, and ground cinnamon until well combined. Set aside.
Cream together the room temperature butter and brown sugar until light and fluffy.
Add the egg and vanilla bean paste (or extract) to the butter and sugar mixture. Mix until fully incorporated.
Fold in the unfed sourdough starter and the mashed ripe banana, mixing until just combined.
Gradually add the dry ingredients into the wet mixture, folding gently until no flour is visible.
Pour the batter into the prepared loaf pan, smoothing the top.
Bake for 55-65 minutes, or until a toothpick inserted into the center comes out clean.
Allow to cool in the pan for 10-15 minutes before transferring to a wire rack to cool completely.
