Preheat oven to 180°C (350°F). Grease and line a loaf pan with parchment paper.
In a large bowl, cream together the butter and brown sugar until light and fluffy.
Beat in the eggs, one at a time, until fully incorporated.
Stir in the mashed bananas, sourdough discard, and kefir until smooth.
In a separate bowl, whisk together all-purpose flour, whole wheat flour, baking powder, baking soda, and salt.
Gently fold the dry ingredients into the wet until just combined—some streaks of flour and small lumps are fine.
Fold in the chopped dates and chocolate chips.
Pour batter into the prepared loaf pan and smooth the top.
Bake 50–60 minutes, until golden and a toothpick inserted in the center comes out clean. If the top browns too quickly, cover loosely with foil halfway through.
Cool in the pan for 10 minutes, then transfer to a wire rack to finish cooling.