In a medium bowl, whisk together the flour, cream of tartar, baking soda, salt, and pumpkin pie spice. Set aside.

In a large bowl, using an electric mixer, cream together the softened butter and 1.5 cups granulated sugar until light and fluffy.

Beat in the egg, then mix in the pumpkin puree until well combined. The mixture might look slightly curdled, which is normal.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix. Fold in the white chocolate chips by hand.

Cover the dough and chill in the refrigerator for at least 30 minutes. This helps prevent the cookies from spreading too much.

While the dough chills, preheat your oven to 375°F (190°C) and line baking sheets with parchment paper. In a small shallow bowl, combine 1/4 cup granulated sugar and ground cinnamon for the coating.

Scoop rounded tablespoons of chilled dough and roll them into balls. Roll each ball generously in the cinnamon sugar mixture until fully coated. The coating should be thick and **shimmering**.

Place the coated cookie dough balls about 2 inches apart on the prepared baking sheets.

Bake for 9-11 minutes, or until the edges are lightly golden and the centers are set but still soft. Do not overbake for chewy cookies.

Remove from oven and let cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely. The **white chocolate chips** will be soft and gooey initially.