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1. Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or non-stick spray to prevent sticking.
2. In a medium bowl, whisk together the all-purpose flour, cream of tartar, baking soda, and salt. Set this dry mixture aside.
3. In a large mixing bowl, use an electric mixer to cream together the softened butter, granulated sugar, and brown sugar until the mixture is light and fluffy, about 3-4 minutes.
4. Add the eggs, one at a time, mixing well after each addition. Then, mix in the vanilla extract until fully incorporated.
5. Gradually add the dry flour mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; a few lumps are okay.
6. In a separate bowl, combine the fresh or frozen fruit with the granulated sugar, lemon juice, and cornstarch. Toss gently to coat the fruit evenly with the sugar and cornstarch mixture. This will help thicken the juices released during baking.
7. Let the fruit mixture sit for about 10 minutes to allow the sugars to draw out the juices from the fruit.
8. Spread half of the snickerdoodle cookie batter evenly in the prepared baking dish. Use a spatula to smooth it out into an even layer.
9. Pour the fruit filling over the cookie batter, spreading it out evenly to cover the entire surface.
10. Drop spoonfuls of the remaining cookie batter over the fruit filling. It’s okay if some of the fruit is still visible; it will spread as it bakes.
11. In a small bowl, mix together the granulated sugar and ground cinnamon for the topping. Sprinkle this cinnamon sugar mixture evenly over the top of the cobbler.
12. Place the assembled cobbler in the preheated oven and bake for 35-40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
13. Keep an eye on the cobbler during the baking process.

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