Combine 3 cups flour, 2 tsp cream of tartar, 1.5 tsp cornstarch, 1 tsp baking soda, 1/2 tsp cinnamon, and 3/4 tsp salt in a bowl and whisk together. In a separate bowl, combine 1/2 cup flour, 1/3 cup packed brown sugar, 1 tsp cinnamon, 1/4 tsp nutmeg, and 1/4 tsp salt. Add 4 tbsp cold butter (cut into 1/2-inch cubes) to this topping mixture and use a fork to break the butter into the dry ingredients until it resembles coarse crumbs with pea-sized pieces. Cover and refrigerate the crumb topping while you prepare the cookie dough.

In a large bowl, beat 1 cup butter with 2/3 cup brown sugar and 1/3 cup sugar for 2-3 minutes until light and fluffy. Add 1 egg, 1 egg yolk, 1.5 tbsp buttermilk, and 1.5 tsp vanilla, mixing until fully combined and smooth. This creates an emulsified, creamy base that will help bind your dough and develop tender cookies.

Add the dry ingredient mixture from Step 1 to the wet mixture from Step 2, stirring until just combined and no flour streaks remain. Refrigerate the dough for 1 hour—this chilling is crucial because it allows the gluten to relax and prevents the dough from spreading too much during baking. I find this resting time makes the cookies hold their shape beautifully and ensures they stay crispy on the outside.

While the dough chills, preheat your oven to 375°F. Pit and slice 3 peaches into 1/2-inch thick wedges. Prepare a parchment-lined baking sheet or ungreased baking sheet. Arrange 12 small mounds of peach slices (3-4 slices per mound) on the sheet, spacing them about 2 inches apart. Top each peach mound with 1/2 teaspoon brown sugar. This creates pockets of caramelized peach that will form the "upside down" base when you flip the cooled cookies.

Remove the chilled dough from the refrigerator and roll it into 12 balls, each roughly 1 inch in diameter. Remove the chilled crumb topping from the refrigerator. Gently flatten and press each dough ball into the crumb mixture, coating all sides generously, then place it crumb-side up directly onto each peach mound on the baking sheet. The crumbs will toast beautifully while baking and add a delightful texture contrast.

Bake for 11-13 minutes until the crumb topping is golden brown and the edges are set (the centers should still feel slightly soft). Remove from the oven and let the cookies cool on the pan for 10-15 minutes—this resting time allows the peaches to set slightly so they hold together when you transfer the cookies. After cooling, gently loosen each cookie with a spatula and carefully flip them onto a wire rack or serving plate so the caramelized peaches face up. I like to let them cool completely upside down to keep that beautiful peach presentation intact.