Preheat oven to 550°F. Lightly grease an 18-by-13-inch rimmed baking sheet with cooking spray. Set aside.
In a large bowl, whisk together the whole wheat flour, all-purpose flour, baking powder, baking soda, salt and cinnamon. In a separate bowl, or large measuring cup, whisk together the buttermilk, eggs, vanilla, and butter. Make a well in the center of the dry ingredient and pour in the wet. Whisk until well combined. Let sit for 5 minutes.
Pour the batter into the prepared baking sheet and spread out into an even layer. Top with banana slices and most of the chopped nuts (save some for topping later on). Transfer the pan to the oven and immediately lower the oven temperature to 425°F. Bake until lightly golden brown, and a toothpick, inserted in the middle, comes out clean, about 14 to 16 minutes.
Remove from the oven and allow to cool down slightly, before slicing and serving with a dollop of whipped cream, reserved nuts and maple syrup. Enjoy!