Preheat oven to 350°F (177°C). Lightly grease a 9×13-inch baking pan. Any 3- to 4-quart baking dish works.
Mix all of the filling ingredients together in a large bowl, then spread into the baking pan. Bake for 10 minutes. Remove from the oven and set aside as you prepare the topping. Keep the oven on.
Whisk the flour, sugar, baking powder, baking soda, and salt together in a large bowl. Using a pastry cutter or 2 forks, cut in the cold butter until the mixture resembles coarse pea-sized crumbles. A pastry cutter makes this step very easy and quick! You could also pulse in a food processor. Pour in the buttermilk, and gently mix until evenly combined. Dough should be slightly sticky once completely combined, but if it’s too dry, add 1 more Tablespoon of buttermilk.
Take handfuls of dough and use your hands to gently flatten out into patties. Place dough all over the top of the warm peach filling. There’s no special trick to this—just flatten the dough in sections and cover most of the peaches.
Brush the top of the biscuit dough with egg wash, then sprinkle with cinnamon-sugar, if using.
Bake for 40–50 minutes, or until the biscuit topping is golden brown and peach filling is bubbling around the edges. To test for doneness, stick a toothpick into the biscuit topping; if it comes out clean, it’s done.
Remove cobbler from the oven, and set the pan on a cooling rack. Cool for 5 minutes before serving. Serve warm with vanilla ice cream or whipped cream.
Cover and store leftovers in the refrigerator for up to 5 days.