Preheat oven to 350 degrees F. Line a baking sheet with parchment paper, and lightly grease with cooking spray. Set aside.

In a large bowl, whisk together the flour, sugar, baking powder, and salt. Set aside. In a separate bowl, whisk together the mashed banana, vegetable oil, vanilla extract, and egg until smooth. Pour into the dry ingredients, along with 1/3 cup of the walnuts. Stir with a rubber spatula until it comes together to form a very sticky dough.

Transfer to a well-floured work surface and divide in half. Shape each half into 8-inch long logs with well-floured hands. Then transfer the logs to the prepared baking sheet and flatten out to 1/2-inch thick logs. Sprinkle the tops with the remaining walnuts and bake for 23 minutes. Remove from the oven, lower temperature to 250 degrees F and let the biscotti cool for 10 minutes. Slice each log diagonally into 10 to 12 slices, returning the slices onto the baking sheet in an even layer, sliced side down. Bake for 15 minutes, flip over and continue to bake for another 15 minutes on the second side. Remove from the oven and let cool completely on a wire rack to cool down completely and dry out a bit. Eat right away, or store in an airtight container at room temperature for up to 3 days. Enjoy!