Start by taking your butter out of the fridge about an hour before baking. You want it soft enough that pressing it leaves an indent, but not so soft it’s melting or greasy.
While your butter softens, whisk together the flour, cream of tartar, baking soda, salt, and all your spices (cinnamon, ginger, nutmeg, and allspice) in a medium bowl. This ensures your spices distribute evenly through the dough later.
In a large bowl or stand mixer, cream together your softened butter and both sugars until the mixture looks light and fluffy – about 3-4 minutes. Don’t rush this step; you’ll see the color change to a lighter beige and the texture become almost cloud-like.
Add your pumpkin puree, egg, and vanilla to the butter mixture. Beat until everything’s well combined, scraping down the sides of the bowl as needed. The mixture might look a bit curdled – that’s totally normal with pumpkin cookies.
Gradually add your dry ingredients to the wet mixture, about a cup at a time. Mix just until you don’t see any more flour streaks. The dough will be pretty soft and slightly sticky – that’s exactly what we want.
Cover your bowl with plastic wrap and chill the dough for at least 2 hours. Chilling helps the flour hydrate and prevents the cookies from spreading too much.
When you’re ready to bake, preheat your oven to 350°F and line two baking sheets with parchment paper. Mix your rolling sugar and cinnamon in a small bowl.
Scoop out dough using a rounded tablespoon (about 1.5 tablespoons per cookie). Roll into balls between your palms – if the dough is too sticky, dust your hands lightly with flour. Roll each ball in the cinnamon-sugar mixture until well coated.
Place the cookies on your prepared baking sheets, leaving about 2 inches between each one. They will spread a bit while baking.
Bake for 11-13 minutes, rotating your pan halfway through. Look for slightly puffed centers and edges that are just set but not browned. The tops will look soft and slightly crackled.
Let the cookies cool on the baking sheet for 5 minutes – they’ll be too soft to move right away. Then transfer them to a wire rack to cool completely.