Preheat oven to 350°F (177°C). Spray a 9x13-inch baking pan with nonstick cooking spray and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using an electric mixer, cream together the butter, brown sugar, and granulated sugar on medium speed until light in color and fluffy (about 4-5 minutes).
Mix in the eggs one at a time, stopping to scrape down the sides of the bowl as needed, then mix in the vanilla extract and mashed bananas.
In a separate mixing bowl, whisk together the flour, baking powder, baking soda, ground cinnamon and salt until well combined.
Alternate mixing the dry ingredients into the wet ingredients with the buttermilk in three additions, starting and ending with the dry ingredients. Stir in the chopped walnuts.
Pour the batter into the prepared baking pan and spread it around into one even layer.
Bake at 350°F (177°C) for 40-45 minutes or until the top is set and a toothpick inserted into the center comes out clean. Remove from the oven and cool completely in the pan on a wire rack.