Preheat oven to 350ºF. Prepare two loaf pans by greasing it or lining them with parchment paper (allowing an overhang on sides). With a fork, beat together mashed banana, zucchini, melted butter, buttermilk and eggs in a large bowl until combined.
In a separate bowl, whisk together flour, sugar, baking powder, baking soda and salt. Add to banana mixture and gently stir until just combined.
Bake for about 55 minutes, until toothpick inserted into the centre comes out clean and top feels springy. Cool for about 5 minutes in pans, then invert into an oven mitt and flip onto wire cooling racks to cool completely before slicing.