Directions

1. In a large bowl, whisk flour, cream of tartar, cornstarch, baking soda, cinnamon and salt until combined; set aside.
2. In the bowl of a stand mixer fitted with a paddle attachment, cream butter, brown sugar and sugar until light and fluffy, 2-3 minutes. Add whole egg and egg yolk; beat until fully incorporated. Stir in buttermilk and vanilla extract.
3. With the mixer on low speed, slowly add dry ingredients, mixing until just incorporated and no dry patches of flour remain, being careful not to overmix. Cover; chill at least 1 hour.
4. Meanwhile, in a shallow bowl, stir together flour, 1/3 cup brown sugar, cinnamon, salt and butter until the mixture resembles coarse, crumbly sand. Chill until ready to bake.
5. Preheat oven to 375°. Line two baking sheets with parchment paper. Sprinkle 1/2 teaspoon of the reserved brown sugar onto the parchment paper; top with 3-4 peach slices. Repeat to make 12 mounds, 2-3 in. apart.
6. Scoop dough by 1-1/2 tablespoonfuls; roll into balls. Gently press each dough ball into the prepared streusel topping; place on top of each mound of peaches, streusel-side up. Repeat.
7. Bake until edges are golden and tops appear set, 11-13 minutes. Cool on pans 10-15 minutes before transferring to a wire rack to cool completely.