Preheat the oven to 350℉ (175℃). Grease an 8x8-inch pan or 2-quart baking dish well with butter and set aside.
In a large bowl, combine 6 cups sliced peaches, ¼ cup dark brown sugar, 1 tablespoon cornstarch, the zest and juice of 1 small lemon, ½ teaspoon pure vanilla extract, ½ teaspoon ground cinnamon, ⅛ teaspoon ground ginger, and ⅛ teaspoon table salt. Gently mix together using a spatula or wooden spoon until the peaches are evenly coated with the spices and sugar.
Spoon the peach mixture into the prepared pan and bake for 15 minutes.
While the peaches are baking, prepare the biscuit topping by whisking together 2 cups all-purpose flour, ½ cup granulated sugar, 1 ½ teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt together in a large bowl.
Cut in ½ cup unsalted butter (cold and cubed), using a pastry cutter, food processor, or two forks, cut the butter into the mixture until it resembles wet sand and the butter is about the size of peas.
Drizzle in ½ cup buttermilk and gently mix until just combined—do not overmix! If the peaches are not ready yet to be topped with the cobbler, refrigerate this mixture.
After 15 minutes of pre-baking, remove the peaches from the oven and dollop the biscuit dough evenly over the peach filling.
In a small bowl, whisk together 1 large egg and 1 tablespoon milk. Brush this mixture over the cobbler dough to give it a beautiful golden color as it bakes, then sprinkle with 1-2 tablespoons granulated sugar.
Return the peach cobbler to the oven and bake for 37 to 42 minutes, or until lightly browned and the biscuit topping is cooked through and golden.
Serve warm along with ice cream or whipped cream if desired.
