Preheat the oven to 350 F (177 C). Lightly grease and line the bottom and two long sides of a 13 x 9 pan.
In another mixing bowl, whisk the flour, cream of tartar, baking soda and salt together until well combined.
2½ cups (300 g) all purpose flour, 1½ teaspoons cream of tartar, 1 teaspoon baking soda, ½ teaspoon salt
In a large mixing bowl, cream the butter with the granulated and light brown sugars together until light and fluffy. Slowly mix in the eggs and vanilla extract until smooth and well combined.
1 cup (226 g) unsalted butter, room temperature, 1 cup (200 g) granulated sugar, ½ cup (100 g) light brown sugar, 2 large eggs (US), room temperature, 2 teaspoons (10 ml) vanilla extract
With the mixer on low or by hand, mix in the dry ingredients until JUST combined. I like to do this ¼ of the dry ingredients at a time.
Drop even spoonfuls of snickerdoodle cookie dough into the prepared baking pan. Using a spoon, flexible spatula or offset icing spatula, smooth the cookie dough until it is in one even layer in the pan.
In a small mixing bowl, whisk the granulated sugar and ground cinnamon together until well combined. Top the cookie dough with the cinnamon sugar.
3 tablepsoons (38 g) granulated sugar, 1 teaspoon ground cinnamon
Bake the snickerdoodle bars for about 20 - 25 minutes. The edges will be set, lightly golden brown and slightly puffed up.
Cool the snickerdoodle bar cookies in the hot pan for 25 - 30 minutes. When the cookies have cooled long enough to transfer out of the pan without breaking, use the excess parchment paper on the long sides of the pan and gently lift the cookie bars out of the pan and to cool on a cooling rack. Slice and enjoy!