Adjust oven racks to upper and lower middle positionsPreheat oven to 400°FLine baking sheets with parchment paper if desired In a medium bowl, whisk together the flour, cream of tartar, baking soda and salt; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, shortening and 298 grams of sugar on medium speed until well combined, 1 to 1½ minutes.
Scrape down the sides of the bowl, add the eggs, and beat again until combined, about 30 seconds.
Add in the dry ingredients and beat at low speed until just combined, about 20 seconds.
Using spatula or large spoon give dough a final mix to catch any stray flour.
If kitchen is warm and/or dough appears a bit 'loose' put dough in refrigerator for 20 minutes or so to firm it up
In a small, shallow bowl, combine the 3 tablespoons sugar and the cinnamon for rolling the dough. Stir or shake well to combine.
Fill a #50 scoop with dough scraping it flat. Eject dough and roll top portion of ball in cinnamon sugar mixture. Don't coat the flat portion of ball. Place on cookie sheet flat side down approximately 2 inches a part.
Bake until the edges of the cookies are beginning to set and the center are soft and puffy, 9-12 minutes, rotating the baking sheets front to back and top to bottom halfway through the baking time. (if using a convection oven the switching and rotating of sheets may not be needed)
Let the cookies cool on the baking sheets 2-3 minutes before transferring them to a wire rack to cool completely.