MELT BUTTER: In a large microwave-safe bowl, melt butter, then let it cool to room temperature. Ensure it's not hot to avoid greasy cookies.
MIX DRY INGREDIENTS: In a large bowl, combine flour, cream of tartar, salt, baking soda, and 3/4 teaspoon ground cinnamon.
ADD WET INGREDIENTS: In a separate bowl, mix melted and cooled butter with white and brown sugar until well emulsified. Add whole egg, egg yolk, and vanilla extract. Whisk until smooth.
COMBINE: Mix the dry ingredients into wet ingredients until just combined, ensuring there are no streaks of flour. Avoid over-mixing. Cover dough tightly and refrigerate it for 1 hour.
PREHEAT OVEN: Preheat oven to 325°F (163°C). Stir together white sugar and cinnamon in a small bowl.
ROLL DOUGH BALLS: Portion dough into 45-gram balls (2 packed tablespoons) and shape them into tall balls. Roll dough balls generously in the cinnamon-sugar mixture. Return shaped dough balls to the fridge for 15-20 minutes, or until they firm up.
BAKE: Place dough balls on a parchment/Silpat-lined sheet with ample spacing (about 6 at a time); bake for 9-12 mins. Avoid over-baking; slight under-baking is ideal! After baking, if any cookies have uneven edges, gently press them inward with a metal spatula. Remove tray from the oven and let cookies rest on the sheet for 5 minutes before transferring them to a cooling rack
STORAGE: Keep cookies in an airtight container at room temperature for 2-3 days or freeze the cookie dough for up to 3 months. (Note 2).
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Notes
Note 1: Proper flour measurement is crucial. Excess flour can lead to dry cookies, while too little can cause spreading. Consider using a kitchen food scale and switch to 'metric' for accuracy. Aim for 395 grams of flour for this recipe.Note 2: Shape dough into balls, roll in cinnamon sugar, and freeze on a sheet pan. Transfer frozen dough balls to an airtight container or bag. Baking from Frozen: No need to thaw; bake frozen dough, adding extra minutes as needed. Bake until edges set and center is soft.