Preheat oven to 375°F (190°C) and line baking sheets with parchment paper.
In a medium bowl, whisk flour, cream of tartar, baking soda, cinnamon, and salt.
In a large bowl, beat butter and sugar until smooth and creamy.
Add egg, egg yolk, and vanilla extract, mixing until combined.
Gradually add dry ingredients to the wet mixture, mixing on low speed until a thick dough forms.
In a small bowl, mix sugar and cinnamon for the topping.
Roll dough into balls (about 1.5 tablespoons each).
Coat each ball in cinnamon-sugar mixture.
Place on baking sheets spaced 3 inches apart.
Bake Soft Thick Snickerdoodles for 10 minutes until puffy and soft.
Gently flatten slightly while warm, then cool on a rack.