In a large bowl, whisk together the brown sugar, butter, pumpkin puree, granulated sugar, and vanilla.
In a separate, medium bowl, whisk together the flour, cinnamon, cream of tartar, baking soda, salt, and baking powder.
Add the dry ingredients to the wet, and mix using a spatula until just combined.
Fold in the pecans if using. Cover the dough with plastic wrap, and place in the refrigerator for 3-4 hours or up to overnight.
Combine the granulated sugar and cinnamon together in a small bowl.
Form dough balls using a 2 tablespoon cookie scoop.
Roll each dough ball in the cinnamon sugar and place on the baking sheet at least 1-inch apart.
Bake at 350°F for 10-12 minutes until the cookies begin to brown around the edges.
Allow the cookies to cool for 2-3 minutes on the baking sheets before transferring to a wire rack to cool completely.
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