Preheat oven to 350 degrees. Spray a 9x9-inch baking pan with non-stick spray or line with parchment paper; set aside.
In a small mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and pumpkin pie spice; set aside.
In a large mixing bowl (with a hand mixer) or the bowl of a stand mixer, beat the sour cream, sugar, melted butter, eggs, and vanilla until combined. Add the zucchini and gently mix in. Slowly add the dry ingredients and mix just until combined.
Spread batter into prepared pan. Combine all of the ingredients for the topping in a small mixing bowl. When large crumbs form, sprinkle the mixture over the batter. Place in the oven and bake for 35-45 minutes or until the center is set. Remove from the oven and allow to cool in the pan for 20 minutes before cutting or removing.
Notes
-You can omit the walnuts completely or swap them out for pecans!