Preheat the oven to 350°F (175°C). Spray a 10-inch cast-iron skillet with nonstick cooking spray. (Alternatively, you can use a 9x13-inch baking dish.)

In a medium bowl, combine the granulated sugar, all-purpose flour, baking powder, kosher salt, and milk. Slowly whisk in the melted butter until just combined. Pour the batter into the prepared skillet.

In a separate saucepan, combine the sliced peaches, brown sugar, cornstarch, and cinnamon. Cook over medium heat, stirring occasionally, for 2-3 minutes until the sugar is dissolved and the peaches are slightly softened. Spoon the peach mixture evenly over the top of the batter in the skillet.

Bake for 45 minutes. To catch any potential drips, you can place a baking sheet lined with aluminum foil on the rack below the skillet.

After 45 minutes, sprinkle the turbinado sugar evenly over the top of the cake. Return the skillet to the oven and bake for an additional 10-15 minutes, or until the cake is golden brown and cooked through.

Let the cake cool slightly before serving. Serve warm, optionally topped with whipped cream or ice cream.