In a medium bowl, whisk together the flour, cream of tartar, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large bowl, use an electric mixer to beat the butter, cream cheese, and sugars on medium-high speed until well combined and smooth, about 3 minutes. Add the egg, egg yolk, milk, and vanilla and beat until combined. On low speed, gradually add the flour mixture and beat until combined.
Press the dough into the prepared pan. Bake for 25 minutes, or until lightly golden brown on top. Place the pan on a wire rack and let cool completely.
Make the topping:
In a large skillet over medium-high heat, melt the butter. In a large bowl, toss all the remaining ingredients together. Add the peach mixture to the hot pan and cook, stirring often, until the mixture thickens, about 6 to 8 minutes.
Top the bars with the warm peach caramel mixture and serve.