Preheat the oven to 350 °F. Place a 12" cast iron skillet into the oven to preheat (if using a 13" x 9" baking pan you don't need to preheat the pan).
In a large bowl, combine 3 pounds fresh peaches with juice from 1 lemon, ¼ teaspoon table salt and 6 oz granulated sugar. Toss to combine. Set the bowl aside for 30 minutes. Meanwhile, prepare the topping.
Whisk together the 5 oz all-purpose flour, 3 ½ oz cake flour, ½ teaspoon table salt, 2 teaspoons baking powder and 2 oz granulated sugar. Toss 4 oz unsalted butter slices into the flour. Use your fingers to work the butter into the flour until the pieces are no larger than a pea.
Whisk 1 large egg and 1 teaspoon vanilla extract into 8 oz heavy cream. Pour the cream into the flour mixture. Toss the mixture with a spoon or spatula until it forms a wet dough.
Pour the peaches into the preheated 12" cast iron skillet or a 13"x 9" baking pan.
Use an ice cream scoop or large spoon to drop dollops of dough over the peaches. Generously sprinkle the top of the biscuits with granulated sugar.
Slide the pan into the oven. Place a sheet of foil on the shelf beneath the cobbler in the oven to catch any peach juice boil over.
Bake until the topping is golden brown and baked in the middle and the juices are bubbling, about 30-35 minutes.
Serve warm or room temperature.