Step 1: Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
Step 2: Blot the pumpkin puree by pressing it between several layers of paper towels to remove excess moisture until you are left with about 1/3 cup of concentrated puree.
Step 3: In a large bowl, cream together the butter, 3/4 cup granulated sugar, and brown sugar until light and fluffy.
Step 4: Beat in the egg yolk, blotted pumpkin, and vanilla extract until well combined.
Step 5: In a separate bowl, whisk the flour, cream of tartar, baking soda, salt, and pumpkin pie spice.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Step 7: Chill the dough in the refrigerator for 30 minutes if it feels too soft to handle.
Step 8: Roll the dough into 1.5-inch balls and roll each ball in the cinnamon-sugar coating mixture.
Step 9: Place on prepared baking sheets and bake for 10-12 minutes or until the edges are set.
Step 10: Allow the pumpkin snickerdoodles to cool on the pan for 5 minutes before transferring to a wire rack.