Preheat oven to 375˚F. Line two large cookie sheets with silicone mats or parchment paper.

In a small bowl, whisk together chai seasoning and sugar for topping. Set aside.

In a medium bowl, whisk together flour, cream of tartar, baking soda, chai seasoning, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl with an electric hand mixer), beat butter and sugar together on high speed until smooth and creamy, about 2 minutes. Add egg, egg yolk, and vanilla, beating on medium-high speed until combined, scraping down the sides as needed. Reduce speed to low and slowly add in the dry ingredients in batches (2 to 3), until combined.

Use a cookie dough scoop to scoop out 1 1/2 Tbsp cookie dough at a time. Roll into balls, then roll in the chai-sugar topping. Place on prepared baking sheets and sprinkle with extra chai-sugar topping as desired.

Bake for 10 to 12 minutes, until the cookies are puffy and soft. Remove from oven and use the back of a spoon to flatten them out as desired. Let cool on the baking sheet for 10 minutes, then remove to wire cooling racks to cool completely.

Cookies will keep at room temperature in an airtight container for up to one week.