Add the flour, cinnamon, cream of tartar, baking soda and salt to a bowl and whisk to combine well. Set aside.
Add the butter, brown sugar and white sugar to a large mixing bowl and cream together with an electric mixture for 3 minutes or until fluffy.
Add the pumpkin and vanilla and beat to combine.
Add ⅓ of the flour mixture into the wet ingredients and beat on low, scraping down the sides of the bowl regularly with a spatula, until combined. Repeat with next ⅓ of the flour, and then the final ⅓.
Cover the bowl and refrigerate for at least 2 hours (or overnight). This is to make sure the cookies are chewy and don’t spread out too much.
Preheat the oven to 350F/175C. Line cookie sheets with baking parchment paper.
Add the rolling sugar and cinnamon to a shallow bowl and stir to combine.
Using a cookie scoop or a tablespoon, scoop out the dough and roll into a ball. Roll the ball in the cinnamon sugar and place onto the baking sheet 2 inches apart. Repeat with the remaining snickerdoodle cookie dough.
Bake for 12 minutes or until risen, golden and the top starts to crack.
Allow the cookies to cool on the cookie sheet for 5 minutes, then transfer to a wire rack to cool completely.
Notes
When mixing the dry ingredients into the wet, be sure not to overwork the dough. This will result in tough and dry cookies.Chilling the dough is very important as it helps to prevent the cookies from spreading too much while baking.Use room temperature ingredients to ensure everything mixes together evenly.For extra flavor, you can add a teaspoon of pumpkin pie spice to the cookie dough or use it instead of cinnamon in the rolling sugar.Don't overbake the cookies! They may look undercooked but will continue to cook as they cool on the baking sheet.