Preheat oven to 350 degrees. Spray an 8 x 8 ovenproof baking dish with non-stick cooking spray. Set aside. Into a large bowl add: oats, pumpkin pie spice, salt, baking powder. Stir to combine. Into a separate bow add: milk, pumpkin puree, vanilla, egg. Stir to combine. Pour wet ingredients into dry ingredients bowl and mix to combine. Pour ingredients into prepared baking dish. Bake for approximately 30-35 minutes or until set. Sprinkle with optional walnuts prior to serving if desired.