In a large microwave-safe bowl, melt butter, then let cool to room temperature. Ensure it’s not hot to avoid greasy cookies.
In a large bowl, combine flour, cream of tartar, salt, baking soda, and 3/4 teaspoon ground cinnamon.
In a separate bowl, mix melted and cooled butter with granulated and brown sugar until well emulsified. Add whole egg, egg yolk, and vanilla extract. Whisk until smooth.
Mix dry ingredients into wet ingredients until just combined, ensuring there are no streaks of flour. Avoid overmixing. Cover dough tightly and refrigerate 1 hour.
Preheat oven to 325°F. Stir together cinnamon sugar coating ingredients in a small bowl.
Portion dough into 45-gram balls (2 packed tablespoons) and shape them into tall balls. Roll dough balls generously in cinnamon-sugar mixture. Return shaped dough balls to the fridge for 15–20 minutes or until they firm up.
Place dough balls on a sheet pan lined with parchment or a silicon baking mat with ample spacing (about 6 at a time); bake 9–12 minutes. Avoid overbaking; slight under-baking is ideal! After baking, if any cookies have uneven edges, gently press them inward with a metal spatula. Remove tray from oven and let cookies rest on the sheet 5 minutes before transferring them to a cooling rack.
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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.Storage: Keep cookies in an airtight container at room temperature 2–3 days, or freeze cookie dough up to 3 months: Shape dough into balls, roll in cinnamon sugar, and freeze on a sheet pan. Transfer frozen dough balls to an airtight container or bag. When baking from frozen, no need to thaw; bake frozen dough, adding extra minutes as needed.