Preheat your oven to 375°F (190°C). Lightly grease a 9x9-inch or similar sized baking dish.

In a large bowl, toss the sliced peaches with 1/3 cup granulated sugar, cornstarch, and cinnamon until evenly coated. Spread the mixture evenly into the prepared baking dish.

In a medium bowl, whisk together the flour, 1/4 cup granulated sugar, baking powder, and salt.

Add the cold, cubed butter. Using a pastry blender or two forks, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.

Pour in the cold milk and stir gently with a spoon just until a shaggy, sticky dough forms. Do not overmix.

Drop large spoonfuls of the biscuit dough evenly over the top of the peaches. It is okay if the fruit peeks through.

Sprinkle the remaining 1 tablespoon of sugar over the tops of the biscuits. Bake for 35 to 40 minutes, until the fruit is bubbling thickly and the biscuits are deeply golden brown. Let cool slightly before serving.