To make the cookie base:
Preheat oven to 350°F and line a 9x13 inch baking pan with parchment paper.
In a large bowl, combine the flour, cream of tartar, baking soda, and salt.
In the bowl of a stand mixer fitted with a paddle attachment or a bowl with a hand-held mixer, beat the butter and sugar on medium high speed until pale and creamy.
Add the egg, mixing until incorporated, and scraping down the bowl as needed. Add the flour mixture in 3 additions, alternating with the milk, and mix just until combined. Spread batter into the bottom of the prepared baking pan (Mixture will be thick. Dollop spoonfuls of batter evenly into the pan, then use a spatula to spread batter to the edges).
To make the gooey layer:
In a small bowl, whisk together the corn syrup, milk, and vanilla.
In the bowl of a stand mixer fitted with a paddle attachment or a bowl with a hand-held mixer, beat the butter and sugar on medium high speed until pale and creamy.
Add the egg, mixing until incorporated, scraping the bowl down as needed. Add the flour and salt in three additions, alternating with the milk mixture, starting and ending with the flour. Spread the batter evenly over the cookie base in the baking dish (Use the dollop-and-spread method here too for easiest spreading).
To make the topping:
In a small bowl, combine the sugar and ground cinnamon to make the topping. Sprinkle evenly over the batter. Bake until top is golden brown, 25-30 minutes. Let bars cool completely to room temperature (the gooey layer won't set until the squares have cooled completely).
Let cool, then slice into 2-inch squares. Store in an air-tight container at room temperature.
Calories: 213kcal, Carbohydrates: 29g, Protein: 2g, Fat: 10g, Saturated Fat: 6g, Cholesterol: 40mg, Sodium: 83mg, Potassium: 48mg, Fiber: 1g, Sugar: 19g, Vitamin A: 323IU, Calcium: 14mg, Iron: 1mg