Preheat the oven to 350°F (177°C). Spray a 9x5 loaf pan with nonstick cooking spray, line the pan with parchment paper, and set aside.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using an electric mixer, mix together the butter, granulated sugar, and brown sugar for 4-5 minutes or until light and fluffy.
Mix in the eggs and vanilla, making sure to mix well after each addition. Stop and scrape down the sides of the bowl, then add the mashed banana and mix until fully combined.
Add the dry ingredients to the wet ingredients and mix until just combined, making sure not to overmix the batter. Gently stir in the chopped walnuts.
Scoop the batter into the prepared loaf pan and spread it around into one even layer.
Bake at 350°F (177°C) for 55-65 minutes or until a toothpick inserted into the center comes out clean. Tent with aluminum foil to prevent excess browning if needed.
Remove from the oven and allow to cool in the loaf pan for 20 minutes. Carefully remove the bread from the loaf pan and transfer to a wire rack to cool completely.
Store in an airtight container at room temperature for 3-4 days or in the refrigerator for up to 5 days.To freeze the the loaf: Wrap tightly with plastic wrap and store in a freezer bag or container in the freezer for up to 3 months. To thaw, place it on the counter for several hours or overnight so that it can come to room temperature.To freeze individual slices: Place the slices of bread on a baking sheet lined with parchment paper, then place the baking sheet in the freezer for a couple of hours or until the slices are frozen. Place the frozen slices in a freezer bag or container and store them in the freezer for up to 3 months. Thaw individual slices to room temperature or reheat frozen slices in the microwave for 20-30 seconds or until warmed through.Butter: If you only have salted butter on hand, reduce the salt in the recipe to 1/4 teaspoon.Eggs: Room temperature eggs are best as they will disperse more evenly into the batter. To bring your eggs to room temperature quickly, place them in a bowl of warm water for 5-10 minutes.Bananas: The riper the bananas, the better! You’ll need about 4 large bananas or 5 medium bananas to get 2 cups of mashed banana.