Preheat the oven to 350°F (180°C). Line a standard loaf pan with parchment paper, leaving a few inches of overhang on each side for easy removal.
Mix the dry ingredients. In a large mixing bowl, combine the flour, sugar, baking soda, and salt. Stir well to distribute evenly.
Add the wet ingredients. Add the mashed bananas, softened butter, milk, vanilla extract, and apple cider vinegar to the same bowl. Mix until just combined - do not overmix. A few small streaks of flour are fine.
Fold in the chocolate chips. Using a rubber spatula, gently fold in the chocolate chips and walnuts (if using). Tip: toss chips in a teaspoon of flour first to prevent sinking.
Fill the pan. Pour the batter into the prepared loaf pan and smooth the top. Press a handful of extra chocolate chips (and walnuts) onto the surface.
Bake for 45–50 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. If the top browns too quickly, loosely tent with foil around the 30-minute mark.
Cool completely in the pan before slicing - at least 30–45 minutes. The loaf continues to set from residual heat as it cools.