Preheat oven to 400° F. spray or grease a 12 inch cast iron skillet with cooking spray or butter.
In a large bowl whisk together the flours, sugar, baking powder, baking soda, and salt. In a separate bowl whisk together the buttermilk, melted butter and vanilla.
Pour the wet ingredients into the dry and fold together with a rubber spatula until the liquid is absorbed (the dough will look shaggy). Dump the dough out onto a lightly floured surface and knead it just until smooth.
In a bowl combine the blackberries, sugar, lemon juice, salt, and vanilla. Sprinkle on the cornstarch and toss together.
Pat or roll the biscuit dough out into a 12x9 inch rectangle. Spread the blackberry filling over the top of the dough leaving a 1/2 inch border all the way around.
Starting at a long side, roll the dough, pressing lightly to form a tight log. Pinch the seam to seal. With the seam side down, cut the log into 12 slices.
Place the rolls cut side down into the cast iron skillet leaving a little space in between them. Brush the tops with the remaining tablespoon of butter then place them on the middle rack of the oven and bake for 20-25 minutes or until they are golden brown.
Remove the rolls from the oven and let them cool for 10 minutes before spreading the cream cheese frosting over the top.
Cream Cheese Frosting
Whisk together the softened cream cheese, butter, vanilla, and powdered sugar until smooth and spreadable. Add more powdered sugar to thicken or a splash of milk to thin the frosting. Spread over the tops of the rolls.