Measure out powdered sugar into a medium sized bowl or plate.
In a large bowl, combine wet ingredients - brown sugar, water, pumpkin puree, syrup and vanilla extract. Whisk together well.
In a separate medium-sized bowl, whisk together all the dry ingredients - vital wheat gluten, salt, baking soda, baking powder, cinnamon and pumpkin pie spice.
Slowly add dry ingredients to wet ingredients and combine. Avoid over-mixing because it will make the dough tough and chewy. Dough should be glossy/wet but not loose. I use a spatula for this step.
Use a 2 tablespoon scooper (or eyeball it) to get the right size. Recipe should make about 24 pieces.
Roll the dough into a ball and press in the palm of your hand until flat.
Dust each side of the cookie-shaped dough in powdered sugar - this is pretty important because it helps keep moisture in our pumpkin seitan. This should just be a dusting; it shouldn't be caked on.
Place on a parchment paper-lined baking sheet.
Bake for about 17 - 19 minutes. The seitan should have fluffed up some and you should be able to poke it without it collapsing.