Begin by preheating the oven to a temperature of 350°F. Meanwhile, prepare a 9x13 inch baking pan by greasing it thoroughly.
In a spacious mixing bowl, amalgamate the melted unsalted butter with both the granulated and brown sugars. Stir continuously until the mixture achieves a uniform consistency.
As the mixture remains in motion, carefully introduce the eggs, one at a time, followed by the vanilla extract, ensuring each ingredient is well-incorporated before proceeding.
In a separate bowl, sift together the all-purpose flour, baking powder, cream of tartar, and salt. Gradually integrate this dry mixture into the wet ingredients, stirring steadily until a homogeneous batter forms.
Transfer the resultant batter into the previously prepared baking pan, ensuring it is spread uniformly across the base.
For the topping, mix the granulated sugar with ground cinnamon in a small dish. Evenly distribute this mixture atop the batter.
Position the baking pan in the preheated oven and allow the batter to bake for a duration of 20-25 minutes. The bars will be deemed ready when their surface takes on a light golden hue.
Upon removal from the oven, permit the Snickerdoodle Cookie Bars to cool for a minimum of 30 minutes prior to slicing and serving.