Line several rimmed baking sheets with parchment paper.
In a medium bowl, combine flour, cream of tartar, baking powder, baking soda, and salt, whisking well.
In a large mixing bowl, combine butter and ⅔ cup sugar. Beat at high speed with a mixer until light and creamy. Add egg yolks and vanilla extract, beating until combined. Add flour mixture, beating until incorporated.
In a small bowl, combine remaining 1 cup sugar and cinnamon, stirring to blend.
Using a levered 2-teaspoon scoop, drop dough into cinnamon-sugar mixture, tossing to coat. Place dough balls 2 inches apart on prepared baking sheets.
Using a finger or the back of a rounded measuring spoon, press an indentation into the center of each dough ball. Fill each indentation with ½ teaspoon apple butter.
Bake for 7 minutes. Remove pans from oven, and fill each indentation with an additional ½ teaspoon apple butter. Bake until edges of cookies are golden brown, 6 to 8 minutes.
Transfer cookies to wire cooling racks, and let cool completely. Store in a single layer in airtight containers, and refrigerate for up to a day.
Recipe by TeaTime Magazine at https://teatimemagazine.com/snickerdoodle-thumbprint-cookies-recipe/