Spray two 8.5 x 4.5 inch loaf pans with cooking spray and line the bottom with parchment paper. Mash the bananas with a fork. Preheat your oven to 350°.
In a large mixing bowl, beat the butter and sugar with an electric hand mixer for 2-3 minutes, occasionally scraping the sides of the bowl with a rubber spatula. The mixture should be creamy and pale.
Add the discard, mashed banana, vanilla, and eggs; beat until smooth.
In a separate bowl, whisk together the flour, salt, baking soda, baking powder, and salt. Add to the banana mixture with any mix-ins. Stir gently just until there are no streaks of flour.
Divide the batter evenly between the prepared loaf pans. Bake in a preheated oven at 350° for about 40-45 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs, no wet batter.
Let the bread cool in the pan for about 10 minutes, then carefully remove the loaves and place them on a wire rack.
You can serve the bread warm, but for the cleanest slices, let it cool completely.