Preheat oven to 325°
Lightly grease a 9×5-inch loaf pan or line it with parchment paper for easy removal. Set aside.
In a large mixing bowl, combine the softened butter, sugar, and vanilla extract. Mix by hand for about 2 minutes, or use a stand mixer or hand mixer on low speed for 1–2 minutes, until the mixture is smooth and creamy.
Stir in the sourdough starter discard, mashed bananas, honey, and eggs. Mix until well combined. A few small lumps of banana are perfectly fine.
In a separate bowl, whisk together the fresh milled flour, baking powder, baking soda, and salt.
Add the dry ingredients to the wet ingredients and gently fold together until just incorporated. Be careful not to overmix, as this can result in a dense loaf. If using chopped nuts or chocolate chips, fold them in now.
Pour the batter into the prepared loaf pan and smooth the top with a spatula. Let the batter rest at room temperature for 10 minutes while the oven preheats to 325°F. This allows the fresh milled flour to fully hydrate for a softer texture.
Bake for 55–60 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs, or internal temperature reaches 200°-205°. If the top begins to brown too quickly, loosely tent the loaf with aluminum foil during the final portion of baking.
Allow the banana bread to cool in the pan for 10 minutes before transferring it to a wire rack. Let it cool further before slicing for the best texture. Serve warm or at room temperature.