Preheat the oven to 350℉. Grease and line an 8x8-inch baking pan with parchment paper.
Prepare the pumpkin puree by squeezing the liquid out (reference photo in recipe for example). Wrap the pumpkin in a few layers of paper towels or a cheesecloth. Gently squeeze out as much liquid as you can with your hands. You may need to replace the paper towels a few times. You will end up with a pumpkin puck that has the texture of soft play-doh.
In a medium bowl, whisk the flour, cream of tartar, baking soda, salt, and spices together until well combined.
In a separate bowl or a mixer, whisk the browned butter, sugars, pumpkin, egg, and vanilla together for 1-2 minutes until very smooth. Add the flour mixture and gently fold together until just combined. Do not over-mix.
Press the dough in an even layer into the baking pan. Make the cinnamon sugar by mixing the two ingredients together and then sprinkle on top of the dough.
Bake for 28-32 minutes, or until the bars are set. Let the bars cool for at least 30 minutes.
To make the icing whisk all of the ingredients together in a bowl. Drizzle over the cooled blondies. Cut into slices and enjoy!