Preheat the oven to 350°F (175°C). Spray an 8 ½ x 4 ½-inch loaf pan with nonstick cooking spray and set aside.
In a medium microwave-safe bowl, melt the butter. Set it aside to cool slightly while you prepare the bananas.
In a large mixing bowl, mash the bananas using a fork or potato masher until mostly smooth with a few small lumps remaining.
Add the melted butter, granulated sugar, egg, and pure vanilla extract to the mashed bananas. Mix well using a whisk or wooden spoon until fully combined.
Add the all-purpose flour, baking soda, baking powder, and kosher salt to the wet ingredients. Use a rubber spatula or wooden spoon to gently stir the batter just until the dry ingredients are no longer visible. Do not overmix.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake for 45 minutes. If the top is browning too quickly, loosely cover the pan with aluminum foil.
Continue baking for an additional 15 to 20 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but no wet batter.
Remove the loaf from the oven and let it cool in the pan for 10 to 15 minutes. Then transfer to a wire rack to cool completely before slicing.