Preheat oven
Preheat to 375°F. Line 2 large cookie sheets with parchment or silicone mats.
Make the Topping
Combine the sugar and cinnamon in a small bowl. Set aside.
Mix the Dry
In a medium bowl, whisk together the flour, cream of tartar, baking soda, cinnamon, and salt.
Cream the Butter & Sugar
In a large bowl, beat butter and sugar on high speed for about 2 minutes until light and fluffy.
Add Eggs & Vanilla
Add the egg, egg yolk, and vanilla. Beat on medium-high until smooth. Scrape down the bowl.
Add the Dry
With the mixer on low, gradually add the flour mixture in 3 parts. Dough will be thick.
Shape & Coat
Roll dough into 1.5 tbsp balls (about 35g each). Roll in cinnamon-sugar topping. Sprinkle extra on top if desired.
Bake
Space cookies 3 inches apart. Bake for 10 minutes - they'll be soft and puffy. Lightly press the tops with the back of a spoon while warm to flatten slightly.
Cool
Let cool on baking sheet for 10 minutes before transferring to a wire rack.