Preheat the oven to 350°F. Line a 9-inch loaf pan with parchment paper so that it hangs over on the two longer sides of the pan, then grease the parchment paper with cooking spray.
In a large bowl, whisk together the flour, sugar, baking soda and pumpkin pie spice. Set the mixture aside.
In a separate medium bowl, whisk together the mashed bananas, pumpkin purée, buttermilk, eggs, melted butter and vanilla. Add the wet ingredients to the dry ingredients and mix just until combined. Pour the batter into the prepared baking pan then liberally sprinkle the sanding sugar on top (optional).
Bake the bread for about 55 minutes or until a toothpick inserted comes out clean. Remove the bread from the oven and allow it to cool in the pan for 5 minutes. Using the parchment paper overhang, lift the loaf out of the pan and on to a cooling rack to cool completely. Slice and serve.