1stick (113 grams) unsalted butter, at room temperature
2ounces(57 grams) cream cheese, at room temperature
1/2cup(100 grams) granulated sugar
1/4cup(50 grams) light brown sugar
1large egg plus 1 egg yolk
1tablespoonmilk
1teaspoonvanilla extract
For the caramel peach topping:
1tablespoonunsalted butter
3cups(420 grams) peeled, pitted, and sliced peaches (from about 4 to 5 peaches – thaw if using frozen)
1/4cup(50 grams) packed light brown sugar
1/4cup(50 grams) granulated sugar
2tablespoonsall-purpose flour
1/2teaspooncornstarch
1/2teaspooncinnamon
1/2teaspoonfine sea salt
1/2teaspoonvanilla
2tablespoonsheavy cream
Instructions
Make the bars:
Preheat the oven to 350°F. Line an 8 by 8-inch metal baking pan with foil, leaving an overhang on all sides.
In a medium bowl, whisk together the flour, cream of tartar, baking powder, baking soda, salt, and cinnamon.
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.