Position a rack in the center of the oven and preheat to 350 degrees F. Melt the butter in a 10-inch cast iron skillet over medium heat, then set aside to cool slightly, 5 minutes.
Whisk the flour, baking soda, cream of tartar and salt together in a medium bowl. Vigorously whisk 1 cup of the granulated sugar, brown sugar and egg together in a large bowl. Whisk in the melted butter. (Do not wipe out the skillet.)
Add the flour mixture and continue whisking until the dough comes together. Let the dough and buttered skillet rest, separately, in a cool place for 30 minutes. (This allows the dough to firm up and the skillet to cool.)
Mix together the remaining 2 tablespoons granulated sugar and the cinnamon in a large bowl. Shape the dough into a large ball and gently roll it in the sugar mixture until evenly coated.
Place the dough ball in the center of the prepared skillet. Bake until the sides are browned, the center is still soft, and the surface of the cookie cracks, about 35 minutes, rotating once halfway through. Cool on a wire rack at least 15 minutes. Cut into wedges and serve warm or room temperature. Store in a sealed container at room temperature for up to 3 days.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
Copyright 2016 Television Food Network, G.P. All rights reserved.