In a medium bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of cream of tartar, 2 tablespoons of cornstarch, 1 teaspoon of baking soda, ½ teaspoon of salt, and 1 teaspoon of ground cinnamon. Set aside.

In a stand mixer, cream together ½ cup of softened unsalted butter, ¼ cup of brown sugar, and ½ cup of granulated sugar until light and fluffy. Add in 1 large egg, 1 egg yolk, ⅓ cup of buttermilk, and 1 teaspoon of vanilla extract, mixing until fully incorporated.

Gradually add the dry mixture to the wet ingredients, mixing on low speed until just combined. Chill the dough in the refrigerator for at least 1 hour.

In another bowl, mix together ½ cup of all-purpose flour, ½ cup of brown sugar, ½ teaspoon of ground cinnamon, and a pinch of salt. Cut in ¼ cup of cubed unsalted butter until the mixture resembles coarse crumbs. Chill until ready.

Preheat your oven to 375°F and prepare baking sheets with parchment paper.

Sprinkle brown sugar on each sheet, then arrange peach slices on top. Scoop and roll chilled dough into balls, press into the streusel topping, and place on the peaches.

Bake for 11 to 13 minutes or until edges are golden. Cool on the pans for 10 to 15 minutes, then transfer to a wire rack.