Step 1

In a mixing bowl whisk together flour, baking soda, cream of tartar and salt for 20 seconds, set aside.

Step 2

In the bowl of an electric stand mixer fitted with the paddle attachment cream together butter, 1 cup granulated sugar and brown sugar until combined.

Step 3

Mix in egg then blend in egg yolk and vanilla. Add in flour mixture and mix just until combined.

Step 4

In a small bowl whisk together 2 1/2 Tbsp granulated sugar with the cinnamon.

Step 5

Scoop dough out 2 Tbsp at a time and shape into balls. Roll each cookie dough ball in the cinnamon sugar mixture to thoroughly coat.

Step 6

Transfer to plates, cover and chill 30 minutes to 1 hour (so cookies spread less). Preheat oven to 350 degrees during last 15 minutes of cookies chilling.

Step 7

Transfer 12 dough balls to a 18 by 13-inch baking sheet lined with parchment paper or a silicone liner, spacing them 2-inches apart (keep remaining chilled).

Step 8

Bake cookies in preheated oven 9 - 10 minutes, or until nearly set but centers are soft (they should appear slightly under-baked).

Step 9

Remove from oven and cool on baking sheet several minutes then transfer to a wire rack to cool. Repeat with remaining dough balls. Store cookies in an airtight container.