Preheat oven to 180°C (350°F).
Line baking trays with parchment paper.
In a bowl, whisk flour, cream of tartar, baking soda, and salt.
In another bowl, cream butter, shortening, and sugar until fluffy.
Beat in eggs one at a time.
Add vanilla extract and mix well.
Gradually combine dry ingredients into the wet mixture.
Stir until a soft dough forms.
Mix cinnamon with additional sugar in a small bowl.
Shape dough into small balls.
Roll each ball in cinnamon-sugar mixture.
Bake for 9–11 minutes until edges are lightly golden and centers remain soft.