Preheat oven to 350°F and grease loaf pan.
In medium bowl mix butter and sugar, add egg, mix to combine, add kefir/yogurt and vanilla, blend well. Add sourdough starter, fold in gently.
In separate, large bowl mix flour, baking soda, and salt.
Make well in flour mixture, pour in chocolate chips, banana, and wet ingredients. Very gently fold together until just combined. It’s okay if some flour is not quite mixed in.
Pour into loaf pan, spread evenly.
Place on middle shelf, in the center. Cook for 55-60 minutes, until golden brown and when toothpick is inserted it comes out clean (besides melted chocolate!).
Pro tip: butter a slice and broil it, best way to eat the best banana bread, or any banana bread!