Heat apple cider in a medium-sized saucepan over medium heat. Simmer until the cider has reduced to 2 tablespoons, about 25 minutes. Let cool for about 15 minutes before using the cider in cookies.
In a small bowl, stir together salt, flour, cream of tartar, and baking soda; set aside.
Using a stand mixer fitted with a paddle attachment or a handheld mixer, beat butter, sugar, and brown sugar until fluffy, about 3-5 minutes. Beat in the egg, vanilla, and the 2 tbsp of reduced, cooled apple cider. Mix until well incorporated, scraping down the sides as needed. Gradually beat the flour mixture into the butter mixture, about ½ cup at a time. Scrape down the sides and bottom of the bowl to ensure everything is incorporated. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or until firm.
When ready to bake, place an oven rack in the middle position and preheat the oven to 375°F. Line a baking sheet with a silicone baking mat or parchment paper. In a small bowl, stir together the apple pie spice-sugar coating ingredients.
Using a #40 (1-ounce) ice cream scoop or measuring spoons, scoop dough into about 2 tablespoon-sized round balls. Roll the balls in the coating, then place them on the prepared cookie sheet about 2 ½ inches apart.
Bake for 12-15 minutes, or until tops are set and edges are golden brown. Let cool on cookie sheets for 2 minutes, then move to a cooling rack to cool completely. Repeat with remaining cookie dough. Enjoy!