MELT BUTTER: Melt the butter in a large microwave-safe bowl and let it cool to room temperature to avoid greasy cookies.
MIX DRY INGREDIENTS:In a large bowl, combine the flour, cream of tartar, salt, baking soda, and ground cinnamon.
ADD WET INGREDIENTS: In a separate bowl, mix the cooled melted butter with both sugars. Whisk until blended. Add the egg, egg yolk, and vanilla, whisking until smooth.
COMBINE: Combine dry ingredients with the wet ingredients until no flour streaks remain, avoiding over-mixing. Cover the dough and chill for 1 hour.
PREHEAT OVEN: Preheat oven to 325°F (163°C). Stir together the white sugar and cinnamon in a small bowl.
ROLL DOUGH BALLS: Portion the dough into 45-gram pieces (~2 packed tablespoons). Encase a Rolo in each, shape into tall balls, and roll in cinnamon-sugar. Chill for 15-20 minutes until firm.
BAKE: Place dough balls on a lined sheet, spacing about 6 apart with ample spacing between each. Bake for 9-12 minutes, aiming for slight underbaking. After baking, if cookies have uneven edges, press them inwards with a metal spatula. Let them sit on the tray for 5 minutes before moving to a cooling rack.
STORAGE: Store cookies in an airtight container at room temperature for 2-3 days or freeze the dough for up to 3 months. (See Note 2).
Notes
Note 1: Flour: Accurate flour measurement is vital: too much makes dry cookies and too little causes spreading. Use a kitchen scale and aim for 395 grams of flour for this recipe. Consider switching to 'metric'.Note 2: Freezing Dough: Form dough into balls, roll in cinnamon sugar, and freeze on a pan. Move frozen balls to an airtight container/bag. To bake from frozen: Bake directly, adjusting time as needed until edges are set and the center is soft.