Preheat the oven to 350 degrees F. Line a metal 9X13-inch pan with foil or parchment and lightly grease with cooking spray. (Glass, ceramic or dark-coated pans may not bake as evenly, consider reducing temperature to 325 degrees.)
In an electric mixer (handheld or stand), add the butter, granulated sugar, and brown sugar, and mix until light and fluffy, 1-2 minutes.
Add the pumpkin, water, and vanilla extract, and mix until well combined.
Add the flour, cinnamon, cream of tartar, baking powder, baking soda, salt, nutmeg, and allspice and mix until just combined and no dry streaks remain.
Spread the batter evenly in the prepared pan. Mix together the cinnamon and sugar for the topping and sprinkle evenly across the top of the bars.
Bake for 20-25 minutes until set around the edges and still soft (but not raw) in the center. Let cool completely before cutting into squares. Serve at room temp or chilled (also delicious lightly warmed with a scoop of ice cream).
Pumpkin Puree: homemade or storebought pumpkin puree works fine in this recipe with this caveat: the recipe was written using Libby's brand since it's probably the most widely available overall. If using homemade pumpkin puree, which tends to be a little more liquid-y, omit the water. Same goes for the Trader Joe's brand of pumpkin puree and other canned brands that are super soft/watery.Flavor: the flavor of these bars improves with time. They are delicious on day one but REALLY good on day two. Keep them well-covered and they will stay fresh for next day eating.Eggs: there are no eggs in this recipe (this helps keep the bars less cakey than other pumpkin recipes with eggs).