Preheat and prep: Heat oven to 350°F (175°C). Line two baking sheets with parchment paper.
Mix dry ingredients: In a medium bowl, whisk flour, cream of tartar, baking soda, and salt.
Set aside.
Cream butter and sugars: In a large bowl, beat the softened butter, granulated sugar, and brown sugar on medium speed for 2–3 minutes until light and fluffy.
Add eggs and vanilla: Beat in eggs one at a time, then add vanilla. Mix just until combined. Scrape the bowl.
Combine: Add the dry ingredients to the wet in two additions.
Mix on low until a soft dough forms. Do not overmix.
Make cinnamon sugar: In a shallow bowl, stir together sugar, cinnamon, and a pinch of salt.
Shape: Scoop dough into 1.5-tablespoon balls (about a medium cookie scoop). Roll each ball generously in the cinnamon sugar to coat.
Space on sheets: Place coated dough balls 2 inches apart on the prepared baking sheets.
Bake: Bake for 9–12 minutes, until the edges are set and lightly golden but centers still look soft and puffy.
Rotate pans halfway for even baking.
Optional churro finish: While warm, lightly brush tops with melted butter, then roll or sprinkle with a fresh coat of cinnamon sugar. This adds a subtle, authentic churro vibe.
Cool: Let cookies cool on the baking sheet for 5 minutes, then transfer to a rack to finish cooling. They’ll deflate slightly and set to the perfect texture.