In a large bowl, whisk together the butter, brown sugar, granulated sugar, pumpkin, and vanilla.
In a medium bowl, whisk together the cinnamon, flour, cream of tartar, baking soda, baking powder, and salt.
Add the dry ingredients to the wet, and mix using a silicone spatula until just combined. Fold in the pecans. Cover the dough plastic wrap and place in refrigerator overnight, or for at least 3 ½ hours. *See notes*
When ready to bake, preheat the oven to 350°F and line two large baking sheets with parchment paper. Mix cinnamon sugar ingredients together in a small bowl.
Portion out the dough into balls using a medium cookie scoop. Roll each ball in the cinnamon sugar and place it on the baking sheet at least 1” apart. If you’d like puffy cookies, do not flatten. If you’d like flatter cookies, press down slightly and add a bit more cinnamon sugar on top.
Bake cookies until they begin to brown around the edges, 10 to 12 minutes. Allow to cool on baking sheet for 3 minutes before transferring to wire rack to cool.