In a medium bowl, whisk together flour, baking soda, cream of tartar, salt and nutmeg.
In a large bowl, whisk together butter, granulated sugar and pumpkin purée until smooth.
Add egg and vanilla and whisk to combine.
Add flour mixture and stir to combine (about 2 minutes).
In a small bowl, combine sanding sugar, cinnamon and allspice.
Drop heaping tablespoons of dough into sugar mixture and roll into 1½-inch balls.
Transfer to parchment-lined rimmed baking sheets, 3 inches apart. Using a spatula, flatten balls to just under ½ inch thick.
Sprinkle with more sugar mixture.
Bake at 375°, rotating sheets halfway through, until light golden and firm to touch (10 to 12 minutes).
Let cool for 5 minutes on sheets, then transfer to a wire rack and let cool completely. Cookies can be stored in an airtight container up to 3 days.