Preheat the oven to 375°F (190°C) and prepare baking sheets with parchment or silicone mats.
Mix the topping by combining sugar and cinnamon in a small bowl.
In a medium bowl, whisk together flour, cream of tartar, baking soda, cinnamon, and salt.
Beat softened butter and sugar in a large bowl until smooth and creamy, about 2 minutes. Add the egg, yolk, and vanilla; mix until combined.
Gradually mix the dry ingredients into the wet ingredients, forming a thick dough.
Roll dough into 1.5-tablespoon-sized balls, coat them in the cinnamon-sugar topping, and place them 3 inches apart on the baking sheets.
Bake for 10 minutes. While still warm, gently press cookies to flatten. Cool on the baking sheet for 10 minutes, then transfer to a wire rack.
Store in an airtight container for up to 7 days.
Enjoy these cookies as a cozy treat for any occasion!